New non-stacking ToastDisplayer implementation

A rehash of PR #27.

This adds an interface for ToastDisplayer.

It deprecates Toaster, but enhances it by upgrading it to use the new interface (without breaking API).

It adds two implementations of ToastDisplayer:
- a regular one, like Toaster, but with the ability to cancel all preceding toasts
- a non-stacking one, which will only show the last toast

if I take `context.getApplicationContext()` from here, that fixes the Collection issue too right? In that, they'll all have a reference to the same (Application) context so it won't be leaking the Activity.