Add support for Google Default Credentials.
Merge pull request #12 from orestica/compacted

Add support for Google Default Credentials.
Modify the OAuth2Credentials.get_access_token() method to return the expiration information, in addition to the access token.
Make OAuth2Crdentials.get_access_token() method return a
collections.namedtuple rather than a dictionary.
Applied some of Craig's suggestions.
Tweak the computation of seconds from a datetime.timedelta object, so
that we consider only the whole seconds.
Add additional docstring for the OAuth2Credentials._expires_in() method,
explaining what values are returned when token_expiry is in the past or
None.
Merge pull request #15 from orestica/get_access_token

Modify the OAuth2Credentials.get_access_token() method.