Color code the growl messages with severity & iPhonesque growl style

The iPhonesque growl style supports up to four different colors when the priority is set to -2, -1, 0 or 1.
http://macthemes2.net/forum/viewtopic.php?id=16783761

These can be set via the $entry['severity'] of the watchdog message.
I've attached a patch that implements this, but also adds in the Firefox icon by default as opposed to the terminal app.

Attached is a screenshot:

FYI: this patch also includes the explicit path bugfix