[Core] Adds general rating entities and interfaces

* Adds RatingForm and Fieldsets
* Adds HeadScriptInitializer (inject scripts into html via config)
* Includes jquery.barrating.js Plugin
* Adds ViewHelper for rendering a rating bar without form.

(references gh-15)
[Applications] Adds rating and comment entities; Adds comment management controller.

* Adds Forms to handle application rating.
[Core,Applications] Adds Comments popup to application detail view.

* Fixes AbstractRatingEntity: infinite loop when calculating average.
* Refactor CommentForm: No need for base fieldset.
* Adds javascrprt handling of comment popup.
* Adds user reference to comment entity

(references gh-15)
[Core] Adds PreUpdateDocumentsSubscriber

Introduces PreUpdateDocumtsSubscriber:
* Allows Entities to change or set certain values prior to persistence.
* Tracks also NEW documents.
* Adds PreUpdateAwareInterface: Entities implementing this will be handled by
  PreUpdateDocumentsSubscriber.

* Adds title attribute to the barrating div generated by the rating view helper.

(references gh-15)
[Applications] Finished comments and rating system.

* Refactors javascript
* Adds create and edit actions to CommentController
* Enhances: Rating display on detail page will refresh after comment creation or editing.

(references gh-15)
Merge branch 'feature/Applications-comments_and_ratings' into develop