Adds custom authentication

Implements OAuth2 bearer token authentication using headers.
Adds package import for OAuth2BearerToken
Removes bad colon .. haha
Added ability to pass headers to get_token()
Added revoke_url. Added revoke_token and refresh_token calls. Consolidate the calls into _make_request to not be repetitive
Merge branch 'master' of https://github.com/sleekslush/requests-oauth2

Conflicts:
	requests_oauth2/oauth2.py