Adding .nuspec, .targets. and .natvis files for VC++ developers

Natvis will allow the majority of the most important Autowiring types to be directly visualized.  This includes the contents of CoreContexts, status visualizations on individual threads, and content inspection of AutoPackets.
Merge pull request #145 from leapmotion/feature-nuget

Add Natvis configurations to allow Autowiring to be visualized in MSVS