Added support for multiple hooks on the same entity

as described in http://facepunch.com/showthread.php?t=1286722
It's backwards compatible with the usual hook functions.
The only compatibility problems that might occur are:
- addons that use hook.GetTable and don't expect subtables
- addons that call hook.Add more than once per function
(the function would be called as often as the hook was added)

In my tests with several addons I didn't find any compatibility problems.