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 think it would be better to check if OAuth was enabled in the config before doing the actual binding of the OAuthInterceptor. No need to bind it if it is not needed.