Tree improvements

fixes #141.

This has lots of improvements for the Tree views

Perf
+ start buffering batch changes to a view
+ start debouncing other things
+ stop re-rendering trees when toggling collapse state

Refactor:
+ Create activityTree class
+ move commands to ActivityNode for build and update

@ianmstewart i think you're going to like this patch :smile_cat: 
your work w/ trees is rock solid.
ha - that's a good point. not really - what you're doing is pretty good minus the fact that it's clobbering da tree obj. will update w/ `_.extend({}, ...)`