Merge pull request #208 from firedrakeproject/remove-cgen

Remove cgen

Replace cgen with the PyOP2 IR everywhere. This also means that all of the kernels which Firedrake produces are purely IR rather than c, except to the extent to which they contain user code.
Some css fixes
Introduce some visual deliniation of classes
remove double title
Merge pull request #214 from firedrakeproject/documentation_css

This branch changes some css to provide better visual delineation of modules and classes in the web documentation.
Initial par_loop draft.

Initial sketch of Firedrake level par_loop.

Needs documentation, testing and debugging.
And now with passing test.
Add extruded test
Documentation
Response to reviewers

* Remove naughty and evil import *
* Spell RW and C correctly.