Implement support for Post field of "full_picture"

Hello,

We had need to use a not-so-documented feature for the /post, /feed and /home endpoints to retrieve the original and/or full sized image from the feed. The feature, while not documented on their API pages (for any version), is presented on the Graph API debugger tool. The field "full_picture" is returned by Facebook if it is provided as part of the "fields" parameter. Therefore, with this pull request the new "Post.getFullPicture()" method will return a value if "full_picture" is added as a Reading field.

Cheers.
Martin