Disable keypress events when panelsnap is disabled

If a panelsnap plugin is disabled (or toggled) any keypress events bound to it continues to cause the panels to snap as you can see in the **[plunker here](http://plnkr.co/edit/K2CqYup8daihYGi4pPw1?p=preview)**.

Not quite sure if this is deliberate but if not I have added the obvious fix.