Stand alone: Move rackets by modifying velocity instead of positions

[tapanij]

That means we also need to use bullet slider constraint so the rackets can't float away after collision when we have more than two players, linear factor doesn't do the trick then.

wiki page: http://bulletphysics.org/mediawiki-1.5.8/index.php/Constraints
how it works: http://www.bulletphysics.org/Bullet/phpBB3/viewtopic.php?f=9&t=2106
how physijs uses it: https://github.com/html5rocks/www.html5rocks.com/pull/366/files
c++ demo: https://gitorious.org/bullet/mainlinemirror/source/b395f81108e8ba1c566fd7ab4ecb316e6fc5e261:Demos/SliderConstraintDemo/SliderConstraintDemo.cpp#L295

Not implemented yet in tundra? https://github.com/realXtend/naali/search?q=sliderConstraint&ref=cmdform