Added functionality to bind spies to concrete instances

This is the work from @durron597 https://github.com/ArcBees/Jukito/pull/24
The only comment I didn't address yet is (from @PhilBeaudoin)

> My preference is to drop all these methods and document the
> bind(Foo.class).toProvider(new SpyInstanceProvider(new Foo()));
> 
> If you keep the methods, rename them
> bindImmutableSpy
> bindImmutableNamedSpy...

because I wasn't sure what you meant