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 meant it as "basic, simple" since I don't really cover the details of the implementation, or what module does this in Theano and how it works, or the fact that not every op in Theano has a C implentation and that you need to adapt your strategy lightly if you encounter that case. 

I understand that, for some, 'crude' has a negative connotation, meaning 'primitive' or 'impractival'. I can replace it with 'simple' or even delete it, if you prefer, I really don't mind.