Presenter refactor

[pnc]

• 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.

[pnc]

There's more: I forgot to remove the (unused??) CGVector definition, which seems like a bad symbol to be exporting.