Multiple interactions

## Allow several actions to occur simultaneously.

For example:
 - Use multiple touches to each perform drags, resizes at the same time
 - Track multiple drags on the same element (eg. multi-finger painting on a canvas)
 - Use the mouse to throw an element into inertia phase and start a new drag while inertia runs on the previous one

Todo:

 - [x] fix gestures
 - [x] fix issues with snapping and restricting
 - [x] allow new mouse interactions while inertia is active on the previous one
 - [x] ~~add `Interactable#maxInteractions` setting or similar.~~ Add per-action `max` and `maxPerElement` settings (3960ad9)
 - [x] ~~add `interact.allowMultipleInteractions` setting or similar~~ `interact.maxInteractions(number)`
 - [x] call pointer event listeners multiple times on different interactions for each corresponding touch in `event.changedTouches`
 - [x] update dr.js comments for new and modified methods
 - [x] decide how to handle autoscroll - single interaction autoscroll works. not guaranteed to work with multiple interactions.
 - [x] modify some demos in the repo to support simultaneous interactions
  - [x] dropzones: draggables work fine but the dropzones need to count the draggables
  - ~~[ ] snap: needs a lot of work~~ too much work for now
  - [x] html_svg: just needs to allow multiple interactions
  - [x] star: just needs to allow multiple interactions

The [star.svg][star], [dropzones][dz] and [html_svg][hs] demos work fairly well with multiple interactions.

Closes #75

[hs]: http://rawgit.com/taye/interact.js/multiple-interactions/demo/html_svg.html
[dz]: http://rawgit.com/taye/interact.js/multiple-interactions/demo/dropzones.html
[star]: http://rawgit.com/taye/interact.js/multiple-interactions/demo/star.svg