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)