app: launch git-cola in the background

Automatically launch git-cola in the background when starting.
We do this by calling ourself in the background with the --nofork
option.  This returns the controlling terminal to the user and
allows git-cola to continue running in the background.

Closes #176

Suggested-by: Gabriel Mazetto <brodock@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>