Tensor predict

This is a first pass at a prediction API for predicting the signal back from a fit of the tensor model. 

Still WIP, but I am putting this here, so that we can continue the conversation started by @stefanv on the nipy mailing list back in June: 

http://mail.scipy.org/pipermail/nipy-devel/2013-June/009025.html

I will email back to the nipy list as well with some more details, but here's the code.