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.
I think the original idea of the `learn` submodule was to keep some special-purpose functions in there that are not to be imported by default. I'm not sure we should change this. Granted, it only has a single function...
What was your rationale of importing it by default?