Add noMultiTouch option to prevent reacting to multi touch events

[cburgdorf]

This adds a new setting noMultiTouch and makes it the default.
With noMultiTouch set to true snap.js won't react to multi
finger gestures. This should be the default as it's quite unlikely
to be what the user expects. It helps to prevent issues
with other gestures like pinch zooming.

[jayd3e]

I would say that Snap.js shouldn't make this an option, it just need to be the native behavior. The gesture we are going for is a single finger swipe to the left or right, no? So if that's the gesture that is supported, we need to stick with that. If we allow a multi-finger swipe, then we need to account for that. I just don't see this as something that should get turned on/off.

[cburgdorf]

That's why I made it the default because I agree that a single finger swipe is what we aim for. I made it an option because I didn't know if people may rely on multi-touch swipes as that's the current behavior.