Add units to Introduction chapter

[paulgoetze]

It might be useful if there were additional units int the Introduction chapter in order to explain more fundamental things.

I thought about adding following ones:

• (Almost) everything is an object (maybe after Hello world unit)
  • Quick overview over the terms object, method, classes,
  • how you can call methods,
  • how you can create new instances of a class,
  • what isn't an object in Ruby
• Variable types (maybe after Assigning Data unit)
  • local, global, class, instance variables
  • how to define them