Added nuget support

Here is my second attempt at including nuget support for gflags. 

It now uses the CMake build system as you mentioned in the previous PR. 

I tried to be less instrusive as possible in the gflags build but I had to modify your CMakeLists in a little so that we can install both Release and Debug libs alongside.

The idea behind nuget package generation is simple. From the nuget folder execute the PowerShell script which will configure, build and install all combinations of VS versions (2010, 2012, 2013), architectures (Win32, x64) and linkage (static, dynamic). 

Then the package is generated with the CoApp tools. I used a SplitThreshold so that we don't end up with many overlay packages that would need to be installed at build time. 

The nuget package can be used with VS as any package or from CMake by installing the package first as you would on *NIX platforms. See provided README for details.