Add CEL unit tests and do some cleanup

This adds several unit tests for CEL functionality and provides the
requisite framework for creating additional unit tests.

This also cleans up some reference leaks that were occurring in
Stasis-Core message callback code.

Review: https://reviewboard.asterisk.org/r/2646/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@393410 f38db490-d61c-443f-a65b-d21fe96a405b