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

yes I guess, just need to double check it shows the toast in all scenarios (got a feeling it won't) I know I'm sorry :tongue:  it should do! but I have some nagging doubt @mr-archano @xrigau @StylingAndroid  , Toast using application context always works?