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 would stop the text after: Theano is able to minimize the number of times C code is called from Python code.

The rest don't seem to provide useful information I think.