Issues with Inelastic collisions

[brandonLi8]

For phetsims/qa#524.

So far, perfectly elastic collisions are working well and I haven't found any bugs (testing on master). I have found a few cases that were incorrect when the elasticity wasn't 100%, particularly when a stack of balls collides with the border.

Example:

[brandonLi8]

This should be fixed in the commits above. I tested the exact same collision in the commit above and found no errors. I also tested all the other scenarios that I found that were broken and they all work fine. This could introduce new bugs. Closing.