NuGetManager: filter supported frameworks to .NETFramework

Before, if both .Net 4.5 and Silverlight 5.0 were supported (as e.g. in
Math.NET Numerics), it would have chosen Silverlight 5.0.
NuGetManager: include compatible references, not just exact matching ones

A package with a .Net 4.0 version does support .Net 4.5 as well.
This was breaking with Math.NET Numerics.
Merge pull request #30 from cdrnet/nuget

NuGet: Fix handling of packages with AssemblyReferences but not PackageAssemblyReferences