Added more unit tests. Cleanup.

Found some edge cases in the code. Tweaked a few things. Can now listen to
the HeaderTokenizer's "done" event even after it has fired, and can listen
to the HeaderParser's "headers" event even after it has fired. In both
cases, the modules will simply call the handler function with the
appropriate arguments.
Total coverage on message parser.
Adding test-email.txt file.
Variable renaming.