Upgrade Umbraco to 7.1.2, remove packages

* Upgrade Umbraco to 7.1.2 via NuGet (+ manually added `interfaces` and `Umbraco.Core` to Core)
* Remove the `packages` folder in favor of NuGet Package Restore.  May want to do some testing to make sure it still builds properly on a fresh clone for everyone.  Some may need to do a "restore" first