Add support for Resource Owner Password Credentials Grant

[alimfeld]

For "first-party" clients (i.e. with a high-level of trust between the resource owner and the client) the OAuth2 "Resource Owner Password Credentials Grant" may be used to exchange credentials for an access token.

Please consider adding support for this grant type.

[jricher]

This can already be enabled using a custom server configuration (such as with a server overlay). Since the password grant is applicable to a very limited subset of use cases (ie, users have passwords, clients are trusted), I'm not inclined to have it on by default in the server. We can potentially refactor the configuration file to make the overlay easier, and you wouldn't have to much with the appcontext directly.

[alimfeld]

Ok... as overlaying the application-context.xml directly is suboptimal, further modularization of the spring config would allow for easier tuning of specific configuration aspects.

[jricher]

Added new issue #568 to track the reconfiguration request, closing this pull request without merge.