Fix `prependChildTo` behaviour

Similar to 589866de63f, prepending elements to another element in
a composite view, should not prepend to all the selectors that match
in the document.

We should use `this.$` instead of just `$` to prepend elements inside the view.

If you see [the test](https://github.com/thoughtbot/backbone-support/blob/master/spec/javascripts/composite_view_spec.js#L106-116) the content gets added to an external container, so I have no idea is this is what you wanted or if this is actually a bug.

For me, a view should not be able to add/remove/edit elements inside other views.