Fix bug in Twoway jumping

Fixes bug in Twoway.  Original code was:
jump = jump || this._speed * 2;

If the user explicitly set the jump speed to zero, it would be reset because of the || operator, leading to unexpected jumping.

I also changed some of the comments to match the current variable names.

Discussed here: 
https://groups.google.com/forum/?fromgroups=#!topic/craftyjs/geUU8qnucTQ