[WIP] Addons using module unification

[mansona]

Todo:

• Dry up lib/broccoli/ember-addon.js (copied from lib/broccoli/ember-app.js)
• Process styles (in some way) from src/ in addon (models/addon.js)
• fix bad error message for Missing template processor when missing ember-cli-htmlbars in dependecies i.e. it exists in dev-dependencies
• broccoli-module-unification-reexporter edge cases?
• throw an error (or do something) when an addon can't decide if it's mu or classic (i.e. has src/ and addon/ or app/ folders)

Tests:

• _addonSrcTree - ember-app.js
• _processedVendorTree - test that it includes stuff from the addon src/ folder
• treeForSrc - models/addon.js
• addons with src/ folder in classic apps

[rwjblue]

Merging into mu-addons branch (aka not master)...