Support NuGet Package Creation

[andreiborodin]

Add support for producing NuGet packages from the generated projects.

[ryantrem]

In the documentation, it says MSB4U cannot currently consume Unity-specific packages like MRTK. @andreiborodin - is this because the MRTK NuGet package's .targets selects the correct dlls based on $(MRTKUnityPlayer), which MSB4U knows nothing about?

If so, then if you wanted to create a Unity-specific NuGet package that worked with MSB4U, would you just hand craft a targets (and nuspec) where the dll(s) are selected based on $(UnityConfiguration) and $(UnityPlatform) instead?

Asking this question in this issue because I think these are all related to MSB4U being able to generate NuGet packages (including .targets) that are compatible with MSB4U NuGet consumption.

@fast-slow-still