feat($compile): explicitly request multi-element directive behaviour

Directives which expect to make use of the multi-element grouping feature introduced in
1.1.6 (https://github.com/angular/angular.js/commit/e46100f7) must now add the property multiElement
to their definition object, with a truthy value.

This enables the use of directive attributes ending with the words '-start' and '-end' for
single-element directives.

BREAKING CHANGE: Directives which previously depended on the implicit grouping between
directive-start and directive-end attributes must be refactored in order to see this same behaviour.

Before:

```
<div data-fancy-directive-start>{{start}}</div>
  <p>Grouped content</p>
<div data-fancy-directive-end>{{end}}</div>

.directive('fancyDirective', function() {
  return {
    link: angular.noop
  };
})
```

After:

```
<div data-fancy-directive-start>{{start}}</div>
  <p>Grouped content</p>
<div data-fancy-directive-end>{{end}}</div>

.directive('fancyDirective', function() {
  return {
    multiElement: true, // Explicitly mark as a multi-element directive.
    link: angular.noop
  };
})
```