replace vendored backbone.js/underscore.js with 'backbone-on-rails' gem
- updates underscore to 1.5.2 and backbone to 1.1.0

backbone had some breaking changes:
- fix url/urlRoot handling in models & collections
- options are no longer attached to the view by default
- collections reset when 'fetch' is called, tell it to keep the existing
  models

other changes:
- fix some events triggering multiple times in connection with deleting
  a model
- use document fragments instead of an element array for stream entries
- adapt jasmine and cucumber specs to the changed code
  * no longer test the backbone router as part of our code
  * jasmine factory already returns model instances, no need to wrap
    that again
Merge pull request #4662 from Raven24/update_underscore-backbone

update backbone.js and underscore.js