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 don't know if I'd be happy with an exception as a user. I think a warning would be more appropriate.

If OAuth is enabled, is it a requirement to have at least 1 consumer/secret pair? How else do I connect?