fix(rxNotify): EOD-281, messages on rxApp level fixed with fade-in

Resolves #453.

Animations will be applied to the `rxNotifications` stack, while the fixed positioning only applies to the default rxNotifications area within `rxApp`. Animations do not occur in the `rxNotification` (singular, not plural) directive, so it should appear fine if it appears in, say, a modal window.

![Screenshot](https://cloud.githubusercontent.com/assets/33945/4852941/2c1194e8-6081-11e4-9197-2380db003288.gif)
NOTE: Because of this line, there will need to be a coordinate effort - I don't think `encore-cloud-ui` or any similar repo will work unless we add `<script src="bower_components/angular-animate/angular-animate.min.js"></script>` in each respective code base. What's the base way to coordinate that?

cc: @parlarjb