Add support for device swipe events

Summary:
Ref T2700. Allow JS to listen for swipes on devices.

There are a bunch of tricky cases here and I probably didn't get them all totally right, but this interaction broadly looks like this:

  - We implement gesture recognition for the mouse in device modes (narrow browser), and for touch events from an actual device.
  - The sigil `touchable` indicates that a node wants to react to touch events.
  - When the user touches a `touchable` node, we start listening for moves. They might be tapping/clicking (in which case we don't care), but they might also be gesturing.
  - Once the user moves their finger/pointer far enough away from the tap origin, we recognize it as a gesture. I hardcoded this at 20px; I wasn't able to find any "official" Apple value, but 20px seems like a common default.
  - At this point, we look at where their finger has moved.
    - If they moved it mostly up/down, we interpret the gesture as "scroll" and just stop listening. The device does its own thing.
    - However, if they moved it mostly left/right, we interpret it as a "swipe". We start killing the moves so the device doesn't scroll.
  - Once we've recognized that a gesture is underway, we send a "gesture.swipe.start" event and then "gesture.swipe.move" events for every move.
  - When the user ends the gesture, we send "gesture.swipe.end".
  - If the user cancels the gesture (currently, only by tapping with a second finger), we send "gesture.swipe.cancel".
  - Gesture events have raw position data and some convenience fields.

Test Plan:
Wrote UI example and used it from the Desktop, iPhone simulator, and a real iphone.

  - The code always seems to get "scroll" vs "swipe" correct (i.e., consistent with my intentions).
  - The threshold feels pretty good to me.
  - Tapping with a second finger cancels the action.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T2700

Differential Revision: https://secure.phabricator.com/D5308