Use cotire to generate precompiled headers, reducing build time

I've integrated [cotire](https://github.com/sakra/cotire) together with the build process to enable precompiled headers.

I'm seeing a ~50% build time improvement on my 2-core laptop (1hr -> 30min or so), and about a 20% build time improvement on my 6-core workstation (20 min -> ~16 min or so). I'm not sure if this might break other architectures, so feedback is appreciated.

Tested on Ubuntu 13.10.

(I'm also not sure if the licensing of cotire would be a problem.)