Add credential provider for NuGet feeds.

Only supports credentials stored in NuGet.config under
the user's profile:

nuget sources update -name MyFeed -username user -password pass
Fix duplicate assembly references when installing NuGet.

References to assemblies in a project that use the qualified assembly name, such as:
"NUnit.Framework, Version=2.6.2.0, Culture=neutral, PublicKeyToken=8cc8392e8503e009"
were not being removed and re-added when installing a NuGet package that included
that reference.