Implement Resource Owner Password Credentials Grant

This is described in [Section 4.3](http://tools.ietf.org/html/draft-ietf-oauth-v2-31#section-4.3) of the OAuth spec.

We want the authentication of the resource owner credentials to be pluggable, so we introduce a new Spring bean for that purpose.  We also supply 2 initial implementations:

* A no-op version that authenticates any username/password given (except for a well known "bad" user).

* An implementation that stores the known resource owners and their credentials in the authorization server's database.

Cleaned up the confirmation of the client credentials a bit since we only seemed to really be doing that fully for the client credentials grant (and we should anytime they are provided).

Added tests for the new code (though more testing could easily be done, I tried to match the level of the existing code).

Addresses issue #21