Zsh message

With the latest update, I would receive the message "Aborting" when trying to start up screenxtv. After looking through the changes, I figured out it was because screenxtv is now calling the zsh shell, which was not installed on my system. These commits do the following:

* check for zsh at runtime and exit with meaningful error message
* update README to let non zsh users know they can still run their favorite shell