Provide an API to disable pinch rotation

[lucaswoj]

fixes #1496

Allows touch / pinch rotation to be disabled via

map.pinch.disableRotation();

and enabled via

map.pinch.enableRotation();

cc @jfirebaugh @mcwhittemore @bhousel @mourner

[jfirebaugh]

The handler Pinch isn't just about pinch, so it should be renamed to Touch, and the example code should read map.touch.disableRotation(). Let's fix this before publicizing this API.

[lucaswoj]

Pinch doesn't encapsulate all touch events, just those that use two fingers. It does not handle panning, for example. Perhaps TwoFingeredTouch would be a more correct name but I think Pinch is ok for now.

Sounds like there's a bigger discussion about refactoring to be had.

[lucaswoj]

@jfirebaugh How's TouchZoomRotate?

[bgentry]

docs could probably also use an update to show how to disable touch rotate (maybe in the existing disable rotation example). Thanks for fixing this!

[lucaswoj]

@bgentry Agreed -- we need to fully document all these interaction handlers and put together an example.

[sheerun]

this would be better as option something like touchZoom: true