Multiple interactions

 - [ ] fix gestures
 - [ ] fix some issues with snapping
 - [ ] add `Interactable#maxInteractions` setting or similar
 - [ ] add `interact.allowMultipleInteractions` setting or similar
 - [ ] add option to allow multiple interactions on the same element
 - [ ] call pointer event listeners multiple times on different interactions for each corresponding touch in `event.changedTouches`
 - [ ] update dr.js comments for new and modified methods
 - [ ] modify some demos in the repo to support simultaneous interactions
  - [ ] dropzone (draggables work fine but the dropzones need to count the draggables)
  - [ ] snap


Closes #75