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.
Can you just tell that this make it easy to do the ref count. Something like:

This function make it easy for your implementation to only support c contiguous inputs. Just call it on your inputs, it do a copy only if needed. The for the refcount, just always decref it after you finish using it.