Added custom bash prompt

Based on the 'my' theme.

This colors the arrow at the start of the second line based on the exit
code of the last command: green for 0, red for non-zero

This functionality currently fails when the 'fasd' plugin is enabled,
since it is messing with the PROMPT_COMMAND function.

![image](https://f.cloud.github.com/assets/907063/1482722/c5d4ba32-46e8-11e3-9648-a7d7c18f8a86.png)