[bug] Disabled buttons can be activated using a keyboard

[Malvoz]

To reproduce:

1. Visit http://danielmontague.com/projects/easyButton.js/v2/examples/#disabling-buttons;
2. Zoom out 1 level so that the button gets disabled;
3. Tab until the "coffee button" has recieved focus;
4. Press Enter.

The expectation is that the button should not be activated in the disabled state.