Make GLPickle more extensible.

Summary of changes
------------------
- Made GLPickle more extensible
- Used the extensibility mechanism to implement things for Model, SGraph,
  SFrame, and SArray.
- Added some additional tests for the same.

Details
-------
Previously, GLPickle could not be extended without making changes directly to
it. Now, we added a mechanism to extend it by implementing 2 simple
methods:

1. __gl_pickle_save__: Save your object to a filename (not handle) given to you.
2. __gl_pickle_load__: Load your object from a filename (not handle) given to you.

As an example, here is a simple class that was extended to be pickled via
GLPickle.

  class SampleClass(object):
      def __init__(self, member):
         self.member = member

      def __gl_pickle_save__(self, filename):
          with open(filename, 'w') as f:
              f.write(self.member)

      @staticmethod
      def __gl_pickle_load__(filename):
          with open(filename, 'r') as f:
              member = f.read().split()
          return SampleClass(member)

Author: srikris

oss_src/unity/python/sframe/__init__.py

@@ -62,6 +62,8 @@
 from .version_info import version
 from .version_info import __VERSION__
 
+from _gl_pickle import GLPickler
+from _gl_pickle import GLUnpickler
 
 class DeprecationHelper(object):
     def __init__(self, new_target):