renamed plugins to contrib (they are not really plugins)
rewrote README.md to reflect new usage
added new dir to setup.py so it gets copied with installation, in views
of making using inventory scripts easier in teh future
The idea is that some plugin would not be called in some
specific case, and the callback should decide by itself.
Having a way to globally disable it is much cleaner than
disabling every method one by one on the plugin side.
My use case is for fedora-infrastructure that cannot be run
from git checkout since it try to connect to the message bus,
but another case would be to bootstrap infrastructure, or to
run the code on a test servers without having all the callback
infrastructure setup.
