Update src/controls.js

Default behavior was this:

jump = jump || this._speed * 2;

In this case, if you explicitly passed a jump of zero, 
jump would still be set to speed because of || operator, 
leading to unexpected jumping ability.

Also changed description to clarify and correct argument name.

Forum discussion here:  https://groups.google.com/forum/?fromgroups=#!topic/craftyjs/geUU8qnucTQ