Nesterov momentum implementation

Based off of the [simplification suggested by Ian Goodfellow](https://github.com/lisa-lab/pylearn2/issues/677) (@goodfeli) and [the corresponding paper by Yoshua Bengio](http://arxiv.org/pdf/1212.0901v2.pdf).

Still need to add tests to the test suite, but I'm looking for a few more eyes on the code to make sure the basics check out. 
[The paper I linked to in the original pull request](http://arxiv.org/pdf/1212.0901v2.pdf).  It includes an alternative formulation of Nesterov momentum that is easier to implement.