Issue #10: Refactor Activiti library class to use OAuth directly

Issue #10
- Refactor Activiti library classes to use OAuth directly
- Stubbed out new health check method for yammer.metrics
- Refactored library unit tests to setup user/groups before class
instead of before each method
- Cleaned up MongoDBHealthCheck
Merge pull request #11 from bloomj/Issue#10_RefactorActivitiOAuth

Issue #10: Refactor Activiti library class to use OAuth directly