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
For protected methods, yes. But there are still methods which are unprotected and an exception would block edge cases such as environments where credentials are being juggled/temporarily disabled (for us: testing).