[xbuild] Add Microsoft.NuGet.Build.Tasks.dll MSBuild task

[xbuild] Add Microsoft.NuGet.Build.Tasks.dll MSBuild task

This is required for interoperating with project.json-based NuGet package configuration files
(not to be confused with DNX or AspNet Core which use it as a project file too) from MSBuild/xbuild.

The task hooks into the build process, resolves assembly references from the nuget package cache
and injects them into the compilation project so the csproj doesn't need to be modified when
adding new references/packages.

It is the first step on our road to PCL5/NuGet3.