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 remind me, we should talk in the python section about how python handle error and defile the PyErr_SetString and PyErr_Format functions and how to find the correspind c name for an error type.