Linear

I've actually been using this backend for a months now. Apart from gradients and the occasional 2-page output it seems to be working. It's on the `linear` branch of `diagrams-backend-tests` if you want to test it.

I suspect most of the problems will be with texrunner which isn't on Hackage yet. Any feedback/testing would be much appreciated :)

http://github.com/cchalmers/texrunner

tested on:
Mac OS X - TeX Live 2014