Attach key handler to map target
Remove noModifierKeys condition for keyboad zoom

Refs #917. With the key hanler now attached to the map target by default we can remove the noModifierKeys condition for the keyboard zoom interaction. This will prevent the back button shortcut (alt + <back arrow>) to work, but it's ok, the map is focused so it's the one with the highest priority.
Add an "accessible" example
Merge pull request #1001 from elemoine/keyhandler

Don't register for keyboard events on the document