Fix module import strategy to avoid ImportError

Exception occurred:
  File "conf.py", line 16, in <module>
    from facebook import __version__
ImportError: cannot import name __version__

Now using the same method as setup.py
Add api.rst and reference it in index toctree
Add documentation on facebook.GraphAPI
Add documentation about the version kwarg
Add documentation for get_object
Add documentation for get_objects
Add documentation for get_connections
Add documentation for put_object
Add code samples for python params and types
Add documentation for put_wall_post
Add put_comment documentation
Add documentation for put_like and put_photo
Add documentation for delete_object
Fix length of header lines
Change documentation section headings.

Since the "Methods" section describes the methods used in the GraphAPI
object, it should be a subsection of the GraphAPI section.
Escape args and kwargs in section headings.
Fix line lengths and whitespace.

See https://docs.python.org/devguide/documenting.html#use-of-whitespace.
Minor grammar and wording changes.
Change Graph API reference link.

Do not link to a specific version in the URL, since Facebook seems to
update the API every few months.
Add API documentation from @mylsb. Fix #186.