Added INITIAL specs/implementation for `setEnd` method.

Spec cleanup is in order.
Significant cleanup/rearrangement of `ContentSelection` spec.

* Added `//= require_tree ./support/spec_support/` to spec/javascripts/spec_helper.js;
* Created new spec/javascripts/support/spec_support/content_selection/describe_an_end.js.coffee “shared spec”, factoring out common `setStart` and `setEnd` method specs;
* Replaced the (largely duplicate) specs for `setStart` and `setEnd` methods in spec/javascripts/content_selection/content_selection_spec.js.coffee with calls to new `describeAnEnd` shared spec.

Next up: `ContentSelection.getContent` method.