Add Form.NestedForm to allow custom overriding template for nested forms

Adds Form.NestedForm and uses it in editors/Object.
NestedForm extends Form but uses a different template which doesn't
have a form tag. This means we're now generating valid HTML.
Merge branch 'master' of https://github.com/powmedia/backbone-forms

Conflicts:
	distribution.amd/backbone-forms.min.js
	distribution/backbone-forms.min.js