Adding new superclass for C ops

This adds a new class (gof.COp) for new C ops to inherit from. It allows to have the C implementation in a different file than the Python implementation and it also takes care of a few C-related methods of the Op class automatically. This pull request also updates the Extending Theano with a C Op tutorial to include an example using this new class.
OK, I understand.
Would it be helpful to still have comments structuring the sections of the file? For instance "Support code", "Apply-specific support code", "Main function"?