Added OAuth support to Gutterball's API

Created from PR #747 against master as gutterball-hibernate branch was merged into master.

- OAuth functionality can be disabled via configuration
- OAuth consumers are added via configuration; similar in format
  to Candlepin's implementation.
- Moved CustomSigner, SecurityHole and RestEasyOAuthMessage from
  server to common
- Added OAuthInterceptor to Gutterball
- Updated the buildfile accordingly
- Added unit and spec tests
I would argue to leave it. When I was writing those up, I was running it manually (though, admittedly under the assumption it would also be run automatically), so that's still an option for the interim. Plus, when we do get the spec tests setup, it'll be ready to go.