Separated token issuing from request authorization. Renamed issue_code_grant to authorize_code_request. Renamed Identity to Client or ResOwner depending on the case.

**Changes the library API, breaks the compatibility with any code using the library**
* Authorizing requests and issuing codes or tokens are now performed by different functions, so issuing must be done explicitly. This allows to choose wether to issue a refresh token or not.
* issue_code_grant is renamed to authorize_code_request because the function doesn't issue a code grant anymore, that must also be done explicitly.
* "Identity" variables are renamed to Client or ResOwner to make it clear wether they contain a client identity or a resource owner identity.
* invalid_client and unauthorized_client have been replaced with the other accordingly to the specification.