drop: fix bugs and add dynamicDrop options
Improve cross-browser compatiblity

interact.js is now IE8 compatible and works more consistantly accross
mobile browsers.
demo: simplify demo and support IE8
demo: tidy css and fix mistakes with svg
Don't change classList of elements
demo: fix css :before element
Use `upEvent` not 'mouseup' for checking click
autoScroll: allow settings autoScroll options

interact.autoScroll(options)

where options's properties are
 - margin: distance in pixels the mouse must be for autoScroll to be
   triggered
 - distance: the distance the window is scrolled by in each axis
 - interval: the time in milliseconds between each scroll of distance

Also use a capital s in autoScroll consistently
demo: separate html, css and js files
Don't try to style the cursor of `document`
Interactable#unbind: stop if no event type array
snap: implement grid snapping for drag and resize
demo/snap: add a funky demo for grid snapping
demo/snap: use <range> input
snap: impelement anchor snapping
demo/snap: Add an interactive anchor visualization
demo/snap: improve controls
demo/snap: fix anchor range check in drawAnchors
snap: disabled by default
snap: fix anchor snapping for range < 0