added new build tasks for development build
added clone method to common, extend now has option to disable deep
added new events class
refactored pair manager, added new events to engine
separated gravity function, applying forces now wakes sleeping bodies
moved collision event triggers
improve sleeping by ignoring inactive pairs
added events demo
added events example to dev demo
added jsdoc for engine events
added version switch for docs
dev builds are now ignored
updated build system, there are now 3 build modes (dev, release, edge)
updated builds, note that "matter.js" is now the edge build
updated readme