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 forgot that props was already covered by the previous tutorial.

You could do the add of 2 vectors and put the code in c_code_support() just to show it?