Implement Handler base class & add tests

Implement HandlerManager & add tests

HandlerManager:
- Keeps track of all handlers
- Provides high-level handler utility methods
- Listens for input events on the map
- Processes input events through all handlers
- Receives & collates handlers' recommendations
  for map transformations, resolving conflicts
- Applies map transformations as appropriate
- Fires output (movement) events as appropriate

[WIP] Replace old handlers/bind_handlers with HandlerManager in Map

Fix touchend case in DOM.touchPos() (touchend may still have touches)

Implement touch handlers

- Add TouchHandler & MultiTouchHandler base classes
- Implement TouchPan, TouchZoom, TouchRotate, TouchPitch Handlers
- Add touch handlers to manager by default
- Add & update tests

Implement inertial easing on moveend; update/add tests

[WIP] Fix some flow errors

Add keydown & keyup events in simulate_interaction.js

Implement KeyboardHandler and add/update tests

- increase allowed tap time diff to 500ms
- increase rotation threshold slightly
- add some zoom thresholding (needs more future work)