Use NuGet for Umbraco references in .Core project

Previously we were adding the interfaces and Umbraco.Core manually, but this doesn't play well with NuGet Package Restore, since it doesn't know there's any packages in use here.  This should fix, just need to remember to remove the unused references when upgrading
Add NuGet reference for RhinoMocks so it restores properly
Merge pull request #590 from tomfulton/upgrade-umbraco

Tidy up NuGet package restore