Add chunk buffer
A few improvements for ChunkBuffer

* insert available items at once
* allow to use pushObject on buffer itself
* add a test using arrayObserver to make sure that it fires only when
  elements are added to arrangedContent
Finish ChunkBuffer implementation and integrate it with Artifact
Fix fetching worker name
Add some debug info
More debug output
Merge branch 'ps-chunked-buffer'
Merge branch 'master' into ps-ember-update

Conflicts:
	assets/scripts/app/models/artifact.coffee
	assets/scripts/app/store.coffee
	assets/scripts/app/views/job.coffee
	assets/scripts/spec/spec_helper.coffee