call gnome-session correctly to start unity

Fix the startunity script to launch gnome-session.

This will fix many of the various bugs caused by gnome-session
not existing during a unity session.

gnome-session requires the DESKTOP_SESSION environment variable
to be set to launch properly. This script should also now be 
adaptable to work with gnome-shell and cinnamon.

Fixes #6
Install unity dependencies on quantal+

Adds gnome-session, ubuntu-settings, ubuntu-artwork, and nautilus
to the Unity target. Removes light-themes, which falls under
ubuntu artwork.
update /etc/timezone on enter-chroot

Sets /etc/timezone in the chroot when entering.

This fixes the clock in Unity, which uses /etc/timezone.