Make e-form more flexible, when there is no parent form element

This little changes allow to bind form controller not only to $parent scope.

For example this will work as expected, and ng-if created scope not absorb "item.amountField" as its own property.
<a editable-number="item.amount" 
	e-required 
	e-min="0"
	e-form="item.amountField"
	ng-if="item.isEnabled">
		{{ (item.amount }}
</a>
<!-- ... -->
<button class="btn btn-default" ng-click="item.amountField.$show()" ng-if="item.isEnabled"> 
Activate
</button>