keyboard handlers patch

[francescocretti]

Fixes #1405.

Keyboard handlers patch:

• event firing added
• possibility of preventing default keyboard behaviour
• possibility of preventing vertical and/or horizontal pan with keyboard

Example of usage:

viewer.addHandler('canvas-key', function (e) {
   e.preventDefault = true; // disable default keyboard controls
   e.preventVerticalPan = true; // disable vertical panning with arrows and W or S keys
   e.preventHorizontalPan = true; // disable horizontal panning with arrows and A or D keys
});

Built and tested with grunt:

>> 223 tests completed with 0 failed, 0 skipped, and 0 todo. 
>> 2051 assertions (in 45948ms), passed: 2051, failed: 0

[iangilman]

Looks great. Just one comment.

Also, please add doc comments in for the event... You can look at other raiseEvent calls in the file to see examples. Here's what they end up looking like once they are processed:

http://openseadragon.github.io/docs/OpenSeadragon.Viewer.html#.event:canvas-click

[iangilman]

I like that we're using just one event name instead of differentiating between key down and key press (which seems like it would be unnecessary in our context). I'm a little concerned that calling it canvas-key-down implies that it's specifically a key down event. Maybe we should call it just canvas-key?

[francescocretti]

Yes I used the same name so it's more "transparent" for the user, that does not need to know about the difference in the backend.

Sure, I agree about changing the event name. ;)

[iangilman]

Now that I think of it, I think we only need one copy of the doc comment, even though it's raised in two places. I'll delete one after I merge this.

Thank you for making this patch; it came together nicely!