Don't create a new event handler every time a handler is registered for a key

[ruidfigueiredo]

This statement:

$(document)
.onLeftArrowKeyDown(previousPage)
.onRightArrowKeyDown(nextPage);

Will create two event handlers for keydown on document.

This could be improved by creating just one event handler for keydown, and eveytime a new key is registered, jQuery's data facilities could be used to store the information about which keys should be handled.

This would also facilitate de-registering an event handler for a specific key (just remove the data information about that key)