added numpy.random.choice function

Hi,

I've added a function that wraps numpy.random.choice to the theano.tensor.raw_random random numbers implementation. I believe this might be interesting in certain applications, e.g. sampling training data (also) without replacement and/or with different probabilities assigned to each training case.

This is my first contribution to Theano so please don't hesitate to let me know if there's anything to improve on.

Sigurd