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.
Symlink /etc/mtab to /proc/mounts to "fix" df. Fixes #253
Store the chroot name and link debian_chroot to it. Fixes #242
Make enter-chroot/unmount-chroot more resilient to shell selection.

Fixes #259.
Improve Debian compatibility. Fixes #257.
Initial Arch conversion.
Fix group list for useradd ($GROUPS is a bash variable).
Arch xephyr/xfce ported and working (detailled checks required).
Small fixes to xfce target.
Ported all targets.
Installer cleanups.
Arch: Make sure /etc/profile is sourced when a command is specified in enter-chroot.

This seems to be a bug in su/bash (See: https://bugs.archlinux.org/task/31831).
Arch: source /etc/profile in enter-chroot.
More proper bootstrap: Use pacstrap instead of manually calling pacman.
It does a better job, and runs install scripts properly.
Advise user what to do if pacman -Syu fails
(like during the /bin->/usr/bin move).
Arch: Add getrelease.sh.
plugdev group does not exist on Arch. sudo is already installed earlier.