Minor code cleanup, and improvements to Message API

I'm working on integrating some patterns from the ZGuide into the API, which I hope will be worth a pull request at some point. But first I wanted to make some improvements to various API elements, starting with Message/RoutedMessage/Frame. Feedback welcome. Some things:

* I changed a few API elements, broke backwards compatibility, and so updated some tests
* I performed a few code cleanups in this commit as well
* Made Frame constructor public but didn't move it to it's own java file

Any problems with breaking backwards compatibility, please let me know.