Updates to NengoGPU Makefile and docs

* Updated NengoGPU Makefile to more gracefully handle specifying which
version of gcc the CUDA compiler nvcc should use. the default is
gcc-4.4; newer versions of gcc often cause problems, especially if
the nvcc version is older.

* Updated NengoGPU docs to reflect the above change as well as the
recent removal of the parallelization UI menu