POC: RESTEasy validator provider

Follow-on to #19 that enables RESTEasy's built-in support for validation of JAX-RS resources without needing Guice method interception (though that's still useful for internal validation). Added a new exception mapper to handle RESTEasy's custom violation reports as their built-in mapper returns a different format (with strange carriage returns that mess up the console when using curl) - to see the original RESTEasy response simply comment out `ResteasyViolationExceptionMapper`. Also the id in the response is slightly different to what you get with @Validate method interception, because the JAX-RS violations don't contain any knowledge about the actual resource class, just the endpoint path and argument.

PS: to get this working without dragging in the whole of CDI/Weld I had to add a dummy (non-exported) type to the siesta-server, otherwise the `GeneralValidatorImpl` from resteasy-validator-provider-11 failed to load.

See https://github.com/sonatype/siesta/commit/40258184f5e364fc6d5c7cc98efea19b779ffeba for the changes made in addition to #19