Further documentation corrections

This PR fixes the following two inconsistencies:

* The docs for save_bearer_token() indicated that the "scope" key in the bearer token dict should be a list, when it is in fact a space-separated list of scopes wrapped in a string (not a list).  The documentation has been corrected.

* The docs for invalidate_authorization_code() indicated that the "code" key should be dict containing the authorization grant, when in fact the authorization grant (i.e. the string) itself is sent through to the function.  The documentation has been corrected.

This PR should close #130.