Allows can-value to be used with contenteditable

This change allows contenteditable elements to be bound to an observable
using `can-value`. It works in much the same way as input[type=text],
      except that in this case changes to the contenteditable are
      observed on blur, since normal elements do not have a change
      event. Fixes #807.
Added more robust isContentEditable function

Applies for all cases in which an element can be editable. For #820
Merge branch 'minor' into can-value-contenteditable

Conflicts:
	view/bindings/bindings.js
Merge pull request #820 from matthewp/can-value-contenteditable

Allows can-value to be used with contenteditable