bug-fix: AuthExpirationTime is now a property & GetToken works

- AuthExpirationTime is made a property, since self.__access.expires is
  a property, according to python keyston client
- GetToken, which internally calls get_raw_token_from_identity now
  passes in (project_id and token) or (username and password) in the
  params along with the authentication url.
Merge pull request #25 from TheSriram/bug_fix

bug-fix: AuthExpirationTime is now a property & GetToken works