Fix some issues with multibindings.

We were crashing in a few places where we either forgot about
multibindings, or assumed SetBinder was the only thing that
could bind a Set of something.

This also drops the AbstractProviderMethodBinding type, replacing
it with just plain 'Binding' and a convention to use the method
name as 'requiredBy'.
Merge pull request #219 from square/jwilson/multibindings_fails

Fix some issues with multibindings.