Tutorial for C op

Work in progress.

This is the first version of the tutorial on extending theano with C ops. I'm looking for feedback from people with experience with it to help check for factual errors as well as from people with little experience extending theano to ensure that the concepts are explained clearly enough for new contributors to Theano. Thanks.
This isn't needed. So I would remove it.

I think it would be great to make another example with an __init__ and __props__ and c_support_code(). And document what __props__ do. I would make a new subsection after this example and put all that extra stuff there.