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.
I think we should keep an example of how to make an error. What could we use for this? Change the example to multiple 2 vectors. That way we need to check if they have the same size.