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.  I think some may need to run the build twice at the moment - can probably fix in the build script, though.