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.
Add ref to c_libraries() and other methods. They are methods, not functions.

I think the syntax for the ref is:

```
:ref:`cop.c_libraries`
```