[web-ui] Adding ability to few runtime stats in graph.

CHANGELOG
=========
* Updated `components/duration_humanizer.js` to optionally abbreviate
  units.
* Added `JobStatsAggregator` component, which takes a collection of
  models that have `stats` and `parents` fields defined, then aggregates
  on a specified stat. Rather fragile and fraught with assumptions at
  the moment.
* Increased the time between polls in
  `components/pollable_collection.js`.
* Merged disparate graph view templates into one.
* Added `active` handlebars helper, which returns "active" if the two
  specified values are equivalent.
* Slightly cleaned up `views/graph_viz_view.js` to incorporate the idea
  of graph decorators. In the long run, `GraphView` and `GraphVizView`
  will simply become decorators. (TODO(AK): Flesh out graph decorators.)