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Plugins work with CPU platform.  This involved separating the reference implementations of plugins into their own libraries.

based on the FindCUDA.cmake that was in the platforms/cuda/cuda-make directory.
Setting of CUDA_NVCC_FLAGS is now in top-level CMakeLists.txt, in case I ever figure out how
to get this working with the cmake 2.8 installed system version of FindCUDA.cmake.
Before this change, there were three separate FindCUDA.cmake areas.  Now there is just one.

Downgraded recent cmake changes to ghastly compliance with cmake 2.4, for the benefit of the simtk dart server.

Export CudaFreeEnergyKernelFactory symbols to avoid link error in test program.

Free energy plugin added
Plugin will not run w/ Obc or GB/VI forces unless line 2004 of gpu.cpp (gpu->sim.totalNonbondOutputBuffers  = 2*gpu->sim.nonbondOutputBuffers;) is commented in -- working on removing this constraint
Also unit tests for GB/VI currently fail