VTimer implementation

This code is largely inspired by JPCSP. Test "threads/vtimers/vtimer" now passes, although it's still not 100% right. I tested it with Xiaolin Showdown mentioned in #501 and it's still not satisfied with what it receives (calls GetVTimerTime and SetVTimerTime in a loop).

Please tell me if you know about any game which uses VTimers and I'll try to test it.