Binding atcommand from script

Added the ability to bind atcommands to NPC events (ex:
NPCNAME::OnEvent?); original version by ToastOfDoom? however heavily
modified by I enabling command level at the invoking/character (@/#)
level and fixes to prevent console errors as well as fixes aimed to
ensure compatibility with rAthena.
Updated the script_commands.txt documentation with the following script
commands: bindatcmd, unbindatcmd and useatcmd.