Change Readme to markdown, add syntax highlighting (#40)

I changed the Readme format to markdown, fixed some typos and added syntax highlighting.

Author: melcha

README.md

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+# Creek - Stream parser for large Excel (xlsx and xlsm) files.
+
+Creek is a Ruby gem that provides a fast, simple and efficient method of parsing large Excel (xlsx and xlsm) files.
+
+
+## Installation
+
+Creek can be used from the command line or as part of a Ruby web framework. To install the gem using terminal, run the following command:
+
+```
+gem install creek
+```
+
+To use it in Rails, add this line to your Gemfile:
+
+```ruby
+gem 'creek'
+```
+
+## Basic Usage
+Creek can simply parse an Excel file by looping through the rows enumerator:
+
+```ruby
+require 'creek'
+creek = Creek::Book.new 'specs/fixtures/sample.xlsx'
+sheet= creek.sheets[0]
+
+sheet.rows.each do |row|
+  puts row # => {"A1"=>"Content 1", "B1"=>nil, C1"=>nil, "D1"=>"Content 3"}
+end
+
+sheet.rows_with_meta_data.each do |row|
+  puts row # => {"collapsed"=>"false", "customFormat"=>"false", "customHeight"=>"true", "hidden"=>"false", "ht"=>"12.1", "outlineLevel"=>"0", "r"=>"1", "cells"=>{"A1"=>"Content 1", "B1"=>nil, C1"=>nil, "D1"=>"Content 3"}}
+end
+
+sheet.state   # => 'visible'
+sheet.name    # => 'Sheet1'
+sheet.rid     # => 'rId2'
+```
+
+## Filename considerations
+By default, Creek will ensure that the file extension is either *.xlsx or *.xlsm, but this check can be circumvented as needed:
+
+```ruby
+path = 'sample-as-zip.zip'
+Creek::Book.new path, :check_file_extension => false
+```
+
+By default, the Rails [file_field_tag](http://api.rubyonrails.org/classes/ActionView/Helpers/FormTagHelper.html#method-i-file_field_tag) uploads to a temporary location and stores the original filename with the StringIO object. (See [this section](http://guides.rubyonrails.org/form_helpers.html#uploading-files) of the Rails Guides for more information.)
+
+Creek can parse this directly without the need for file upload gems such as Carrierwave or Paperclip by passing the original filename as an option:
+
+```ruby
+# Import endpoint in Rails controller
+def import
+  file = params[:file]
+  Creek::Book.new file.path, check_file_extension: false
+end
+```
+
+## Parsing images
+Creek does not parse images by default. If you want to parse the images,
+use `with_images` method before iterating over rows to preload images information. If you don't call this method, Creek will not return images anywhere.
+
+Cells with images will be an array of Pathname objects.
+If an image is spread across multiple cells, same Pathname object will be returned for each cell.
+
+```ruby
+sheet.with_images.rows.each do |row|
+  puts row # => {"A1"=>[#<Pathname:/var/folders/ck/l64nmm3d4k75pvxr03ndk1tm0000gn/T/creek__drawing20161101-53599-274q0vimage1.jpeg>], "B2"=>"Fluffy"}
+end
+```
+
+Images for a specific cell can be obtained with images_at method:
+
+```ruby
+puts sheet.images_at('A1') # => [#<Pathname:/var/folders/ck/l64nmm3d4k75pvxr03ndk1tm0000gn/T/creek__drawing20161101-53599-274q0vimage1.jpeg>]
+
+# no images in a cell
+puts sheet.images_at('C1') # => nil
+```
+
+Creek will most likely return nil for a cell with images if there is no other text cell in that row - you can use *images_at* method for retrieving images in that cell.
+
+## Remote files
+
+```ruby
+remote_url = 'http://dev-builds.libreoffice.org/tmp/test.xlsx'
+Creek::Book.new remote_url, remote: true
+```
+
+## Contributing
+
+Contributions are welcomed. You can fork a repository, add your code changes to the forked branch, ensure all existing unit tests pass, create new unit tests which cover your new changes and finally create a pull request.
+
+After forking and then cloning the repository locally, install the Bundler and then use it
+to install the development gem dependencies:
+
+```
+gem install bundler
+bundle install
+```
+
+Once this is complete, you should be able to run the test suite:
+
+```
+rake
+```
+
+## Bug Reporting
+
+Please use the [Issues](https://github.com/pythonicrubyist/creek/issues) page to report bugs or suggest new enhancements.
+
+
+## License
+
+Creek has been published under [MIT License](https://github.com/pythonicrubyist/creek/blob/master/LICENSE.txt)