Added NVTT library to cmake files.

When building osgEarth with OSG 3.2.1-rc2 on windows (VS2012) I was having issues with the nvcore.lib library not being found.  It appears osgEarth.dll and osgdb-osgearth-gdal need this library.  I've modified the CMake files using a slightly modified version of FindNVTT.cmake from OSG to prevent having to add the libraries manually in VS.

May also need to update the documentation to indicate that the NVTT libraries are required.

BTW, this is my first time using git and github so hopefully I'm do this correctly.  I have additional changes I'd like to contribute and I'm using this as a small test of the process.