Presenter refactor

* Factored notification queueing logic into `JCNotificationCenter`.
* `JCNotificationCenter` has a `presenter` property. It uses this object (an instance of `JCNotificationBannerPresenter`) to present notifications it receives. Whenever it has a new notification, it calls `presentNotification:finished:` on the current presenter. The presenter does whatever it wants and calls the `finished` block when the user taps or the banner times out.
* Factored presenter view creation into helper functions so it's easier to write your own presenter subclass. (Probably needs more, but that's beyond the scope of this PR.)
* Made some of the sizing of the Android-style notification configurable as properties.