Adding scopes to collections

This creates behavior discussed in https://github.com/lox/pheasant/issues/115. Our use case is summed up in:

https://github.com/lox/pheasant/issues/115#issuecomment-46605967
Should we throw a method-not-found exception in an `else` block here? Otherwise we'll be silently swallowing errors (or rather, returning `null` from missing methods).