Add AuthMethod support to getting tokens.

Adds 1 new method to the AuthMethod interface, get_token, that returns
the ApplicationGrant that matches the provided token value and service
for the AuthorizationMethod. Also adds 2 new decorators that utilize
the AuthorizationMethod.get_token method to convert passed in tokens to
API calls to the appropriate ApplicationGrant.

Signed-off-by: Tyler O'Meara <Tyler@TylerOMeara.com>