Add a shell helper for demos/presentations

This will keep your bash/zsh prompt at the top of the screen.
It's useful for presentations and demonstrations.

Other features:
* clears the screen automatically with each command run
* lists directory contents when entering them (abbreviated if needed)
* shows the git or svn status in directories that have them

Run the commands autotop and unautotop to enable or disable.
Early bailout if not on bash

Doesn't support Zsh yet
Merge pull request #319 from binford2k/feature/alwaysontop

Add a shell helper for demos/presentations