Adding "idan" theme

A functional, uncluttered theme. Boasts the following features:

Left Prompt:
* Current virtualenv (if applicable)
* Truncated CWD (just the current folder's name)
* Git branch and dirty state (if applicable)

Right Prompt:
* Full CWD path
* Last error code (if applicable)
Add a newline before prompts
Merge pull request #31 from idan/idan