Make Project aware of addons.

* Add `Project.prototype.dependencies` as a list of all dependencies &
  devDependencies in the projects `package.json`.
* Add `Project.prototype.availableAddons` as a list of all dependencies
  that contain the `ember-cli-addon` keyword.
* Add `Project.prototype.addons` as an array of instances of each addon
  found.
  * The addon package is required and a new instance is created.
  * The current project instance will be passed to the addons
    constructor.
* Update preprocessors to use the project's dependencies.
Merge pull request #1012 from rjackson/addons

Make Project aware of addons.