Migrate to NuGet Automatic Package Restore

According to the following pages, NuGet has changed their recommended approach to restoring packages:

* http://docs.nuget.org/docs/reference/package-restore
* http://docs.nuget.org/docs/workflows/migrating-to-automatic-package-restore

This pull request implements the described migration to using Automatic Package Restore.

While command line builds must now explicitly invoke **nuget restore** before building a fresh clone, the usability problems prior to #54 (where builds within Visual Studio also required an explicit package restore) will not suffer from the same problem because the **NuGet.targets** has now been removed.

The **NuGet.Config** file remains in place so the handling of the **packages/** folder does not change.