This replaces the fedmsg message publication with a plugin-based approach.
Plugins can be specified using setuptools entry-points and must extend an
abstract base class defined in resultsdb.messaging.
The original fedmsg publication code is reimplemented as a new plugin and a new
DummyPlugin is added for use in the test suite. The original test cases all
still pass. Two new test cases are added that use and verify the DummyPlugin.