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

ref: https://github.com/novoda/sun-mobile-android/blob/develop/account-manager/src/main/java/com/newsuk/sunplus/SunPlusAccountAuthenticator.java#L81
I don't think the `Context` in there is an `Activity`, and it works :)