Add Python 3.x support through Cython

It's now compatible with both Python 2 and 3. I haven't used setuptools because I didn't know how to add Cython support.

If you have time, please update the sparsesvd package in pypi. There is no alternative I'm aware of for doing SVD on sparse matrices in Python 3.x (divisi2 doesn't support 3.x, and scipy.sparse consumes a lot of memory suggesting it's converting to a dense representation). Feel free to discard changes you don't like.