make cudamat pip installable.

This set of changes converts cudamat into a typical python package so it can be installed via `pip` or `setup.py install`.

Admittedly, this has only been tested against Linux and OSX based environments to date.
This seems like a hack, but given that distutils does not directly support compiling CUDA code and forcing it to do so seems to be [much hackier](https://github.com/rmcgibbo/npcuda-example), I guess we could leave it at that. It should work in all cases in which the old approach worked, right?
What I like about this imperative approach is that it would easily allow us to detect the architecture of all installed GPUs before compiling cudamat.