missing destroy method (to remove PREVENT_DEFAULT_KEY_CODES for example)

[jpsarda]

When removing a game dynamically (with javascript) from the dom, the keydown events on the document (among others I guess but I didn't check all of them) are not removed. The consequence is for example that the space bar (which is in the list of PREVENT_DEFAULT_KEY_CODES) doesn't work anymore in input fields for example.

We miss a destroy method that would take care fo all events unbind.