Create SentryStub to delegate sentry operations

Instead of using different implementations of the AuthValidation and
the Unmarshaller, regroup everything in a singleton SentryStub
which will do most of the Sentry operations and will store
details on the received events
Mark javax.servlet-api as provided
Add a removeEvents to the stub
Create a servlet to access the details of sentry stub
Simplify code
Do not log a warning when UDP is closed
Create a SentryStub client for ITs
Create integration test for log4j
Run integration-tests (separatly) in travis