add a lua call to limit a pilots maximum speed

Rewrote my first code according to your suggestions (see http://colabti.org/irclogger/irclogger_log/naev?date=2013-08-26#l67)
This code no longer sets pilot->solid->speed_max directly
Speed is now set in px/s instead of %speed
if you set the speed to 0 px/s the limiter is cleared