Add GNOME target

Add a GNOME target to crouton, and factor out common code to
a reentrant wrapper around gnome-session.
Improvements to gnome-session-wrapper

Add optional session argument
Ensure style is consistent with the rest of crouton
Pass gnome and ubuntu as args to wrapper script
Merge pull request #281 from smibarber/gnome

Add GNOME target
Add mechanism for controlled restarting of the setup script.

This allows for the environment to be reset after bootstrapping takes place.
Don't hardcode dbus-daemon path. Thanks to @smibarber's #282.
Remove croutonticks, as it is no longer necessary.

https://gerrit.chromium.org/gerrit/#/c/45317/ has trickled down to stable.
Check if make_dev_usb has run; if so, enabling vboot bricks the system.
Fix croutoncycle on some systems.
Make gnome-session-wrapper executable in the repo.
Get rid of crusty old intentionally confusing README language.
Special-case the internal trackpad in croutonwheel.

Fixes #149, although additional work is needed to support external trackpads.
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.