Don't version endpoint twice.

utils.get_application_graph sets version, passes it to GrapAPI from facepy which applies it in _get_url.
Throws OAuthError: [803] (#803) Some of the aliases you requested do not exist: v2.1
Allow setting custom Facebook API version.
Bump version.
Refactor get_application_graph.
Merge pull request #31 from riklaunim/master

API versioning fixes and configuration options