Spelling
Add SoundCloud to the providers not supporting Auth headers
Merge pull request #43 from mjibson/master

SoundCloud and spelling fixes
Update metadata server URL to be a FQDN.

Without a fully qualified domain name, containers (like Docker) can't
connect to the metadata server. Update the address for the metadata
server to be a FQDN so containers can use the library. See #44.
Merge pull request #45 from secondbit/master

Update metadata server URL to be a FQDN.
Support token expiration for Facebook OAuth 2.0.

Facebook's OAuth 2.0 implementation seems to be broken and
returns expires_in value in expires. Fallback to expires field to
handle the expiration time for Facebook.
Handle non-2xx for metadata server responses.
Allow users to get extra fields from a token response.
Locally cache oauth tokens.
This change is for both App Engine and Managed VMs so that these apps can scale without running into the app_identity_service quota limit due to calling appengine.AccessToken too frequently.  An added benefit of caching is that calls to Google APIs will be significantly sped up due to removing the round-trip calls to the api_identity_service.