Allow Blueprints to add packages to projects package.json.

* Add `Blueprint.prototype.addPackageToProject`.
* Add slow smoke test to confirm `ember g http-proxy` adds packages.
* Removes unneeded packages from ember-cli's package.json.
Merge pull request #1598 from rwjblue/allow-blueprints-to-add-packages

Allow Blueprints to add packages to projects package.json.