Add init and remove events

[ricardobeat]

This patch adds etch:init and etch:remove events which are triggered after editableInit and before removing.

Use case: the editable area has a draggable attribute which needs to be unset during editing, and put back when done.

Alternatively the events could be fired on the target element instead of the model, but since etch is already tightly coupled to Backbone I thought I'd follow that path.