Add support for GradientDescentOptimizer

[chrhansen]

I'm not sure if this is strictly necessary for the 0.1.0 Milestone, however having at least one optimizer/trainer would be great.

Python documentation:
https://www.tensorflow.org/versions/r0.10/api_docs/python/train.html#GradientDescentOptimizer

Python implementation:
https://github.com/tensorflow/tensorflow/blob/master/tensorflow/python/training/gradient_descent.py#L27

It's probably a good idea (like for most of our other implementations) to follow the design used in the Python API. In the case of GradientDescentOptimizer it looks like they've built an abstract Optimizer-class that GradientDescentOptimizer (and a few other optimizers) inherit from.

[nethsix]

I've looked at this on-off a bit. I will try to work on this and see how it goes.