Instead of having every model which might need a provider having to
specify every possible provider as input argument, we now have a plugin
model which can expose a list of providers, which the event manager then
will notify about when they are loaded.

Conflicts:
	public/res/publisher.js
	public/res/sharing.js

For some reason, http:https://github.com/RobinThrift/gulp-requirejs is gone (404), but if you use http:https://github.com/bornkiller/gulp-requirejs
then it works.

This fixes issue benweet#596"