Added OAuth support to Gutterball's API

- 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
Merge pull request #756 from candlepin/crog/gb-oauth

Added OAuth support to Gutterball's API