Make fedmsg-notify distribution-agnostic

This pull-request makes fedmsg-notify extensible for use by other distributions.

Two main steps to do so:
 - Making the topic prefix configurable
 - Splitting out the fedora-specific stuff in a module, with stubs if the import fails

I then added the equivalent of the Fedora mechanisms in a Debian-specific module.

Obviously, it works on a Debian system.
![It works!](http://perso.crans.org/dandrimont/fedmsg-notify.png)

I don't have a fedora system at hand, so please let me know if stuff is broken and I'll fix them.