Fix using a NuGet 3.0 package source.

NuGet addin needs to reference NuGet.ShimV3.dll and register a shim
controller on startup for the package sources.

Also requires the following binaries:

Newtonsoft.Json.dll
NuGet.Versioning.dll

Issues:

 1. Package source changes also need to be registered with the shim
controller. May have to use a single instance PackageSourceProvider.
 2. Packages left behind in the packages directory, but not referenced
by any project, seem to be re-downloaded each time the Manage Packages
dialog is opened.
 3. Standard default package icon is not displayed.