Colourful PS1

Make the BASH prompt look prettier. Tested on OSX Mountain Lion and Linux Mint 14.

![Linux Mint 14](https://f.cloud.github.com/assets/270946/16077/1e86999a-4800-11e2-8952-4ea862676edf.png)
![OSX 10.8.2](https://f.cloud.github.com/assets/270946/16078/1e95db12-4800-11e2-8be1-21936a79a155.png)