Update src/controls.js

Old code was this:

jump = jump || this._speed * 2;

This meant that when the user explicitly set the jump argument to zero, it would be re-set to the speed, creating unexpected jumping.

I also updated the comments/description to reflect the current parameter names and behavior.

Discussion here:
https://groups.google.com/forum/?fromgroups=#!topic/craftyjs/geUU8qnucTQ