Switch to a twisted.plugins based plugin loader.

Removes the ability to swap out old code for new code, since that was a
property of exocet.

Based on @uberj's patch at
https://github.com/uberj/hamper/commit/87ad26647683038a28713eb495e7b5169698eaab

changes. Lines starting
Remove unused bravo and exocet dependencies.
Cleanup.