Add support to return cuda arrays to python

[chaithyagr]

This is added support to #74 .
Once we have #74 in, we can work on cuda_array_interface : http://numba.pydata.org/numba-doc/dev/cuda/cuda_array_interface.html

The idea is that if the user wants, he can send Data on GPU and obtain back data in GPU.
This can be used for faster reconstruction, given that we can reduce the to and fro from CPU to GPU and vice versa.

This is comparatively low priority for me, but I can work on this once I have some time