Colinmacdonald/487 fix container

#1 Needs to be merged so the example in this PR works.

Renamed the "container" to wrapper.
 - The container will be an element provided in `options.container` or the `document.body`.
 - The wrapper is now a child of the container that holds the notifications.

The container is never removed.
The wrapper is only removed at #destroy and is hidden when empty.