C++ wrapper for CUDA class SP

[breznak]

with aim to run cudaSP seamlessly from C++. Ideally a drop-in replacement.

• integrate HelloSP & SpatialPooler.cu into 1 file
  • there should be SpatialPooler.cu & a main.cu (main only calls initialization & compute of SP.cu)
• cmake detect CUDA, nvcc & nVidia HW availability
  • cmake compile using nvcc & run tests
• C++ wrapper
  • wrap all CUDA code with ifdef HAS_CUDA
  • SpatialPoolerCuda.hpp that wraps implemented cu code to c++ API (minimal initialize & compute)
  • SpatialPoolerCuda.cpp implements simple buffering
    • as c++ compute calls with a singleton data, while cu calls with block of data
  • cmake builds c++ wrapper