Added support for using different versions of Graph API (Includes Tests)

This adds support for using different versions of the Facebook Graph API.
 - Implementation expects a specific format and handles error with an explantation on format.
 - Facebook fails silently by defaulting to v1.0 when an invalid version is given. In this implementation, an explicit error is thrown when the user tries an invalid version.

It also adds a method to check what version of the Graph API is being used, by looking at the `facebook-api-version` attribute [Facebook is now including with *all* calls of the to the Graph API](https://developers.facebook.com/docs/graph-api/using-graph-api/v2.1#debugging). This attribute was added with the [2.1 update](https://developers.facebook.com/docs/apps/changelog#v2_1_new_features).

Tests have been added to verify that the correct version of the Graph API is being used.
Could we use `valid_API_versions` in the exception message so that adding a new version does not require modifying multiple lines of code?