more improvements on cffi/opencl backends; also moves the contents of gravity.py into extensions.py.
Merge branch 'feature' into dev
avoid an annoying UserWarning when using tox.
now cffi/pyopencl cache_dir name prefix is set to ~/.tupan.
Merge branch 'feature/ext' into dev
Merge branch 'master' of https://github.com/GuilhermeFerrari/Tupan