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.
Are you sure, the PyObject is a wrapper around another c object? I think it is an object, with some extra fields that python need, like the refcounts. In some case, it can countain just a pointor to another object, but I don't think this is general and I don't think it is the case for ndarray. @abergeron can you confirm?