naughty: notify() returns notification object

This makes notification mangling outside of naughty.lua much easier:
http://awesome.naquadah.org/wiki/index.php?title=Naughty#Popup_calendar

Signed-off-by: koniu <gkusnierz@gmail.com>

lib/naughty.lua.in

@@ -74,6 +74,7 @@ local counter = 1
 -- @field height Popup height
 -- @field width Popup width
 -- @field die Function to be executed on timeout
+-- @field id Unique notification id based on a counter
 -- @name notifications[position]
 -- @class table