Merge pull request #374 from hypothesis/extend-adder-2

Split the adder plugin to 3 parts
Align Delegator with Backbone.View

Take an opportunity to align our idioms where we have existing patterns
that are very similar to Backbone and to move away from the
`Annotator.Plugin` pattern with new UI components.

- Invert the order of text in Delegator event hash keys to match the
  call signature of jQuery's `.on()` instead of `.delegate()` and
  the way the feaure works in `Backbone.View`. `Annotator.Plugin` is
  changed to preserve bw compatibility, since it already handles compat
  and deprecation warnings for non-DOM events.
- Use `this.el` and cached `this.$el` instead of `this.element`. The
  latter is preserved for bw compatibility. Add `this.$` as an alias
  to `this.$.bind(this.$el)`.