This updates the login methods on the server and transport classes to allow

logging in to the Review Board server with either a username and password

combination or an API token.

Ran unit tests.
Tested with the upcoming web based login flow change which uses an API

  token to login.
Tested logging in with a username and password.
Tested logging out.
Tested passing --username and --password options to commands.
Tested passing --api-token option to commands.