Have Brocfile.js export a function

[oligriffiths]

This PR implements the basics needed for Brocfile to return a function by default, and wrap existing brocfile exports in a function for backwards compatibility.
The intention for this is to make Broccoli work the same as Ember-CLI (to avoid confusion), and it also paves the way for us to pass in a configuration object containing things like environment and other build-time useful info (as Ember-CLI does).

One thing to note, is

broccoli/lib/index.js

Lines 7 to 9 in b5e4c8b

	get loadBrocfile() {
	return require('./load_brocfile');
	},

would now return a function rather than a node. This obviously changes the API, but I don't know if that's an issue or not given there was no test for this else it would have failed.

[stefanpenner]

Lgtm

[oligriffiths]

Dope. Thanks guys