Reuse Prototype constructor
Repurpose .extend() to extend an existing class/instance in-place

The prevoius function created a subclass from an existing class. The new
functionality is more useful, since there's no nicer way to do it. It's also
probably better to only have one way to subclass things.
Pass null to bound/delegate instead (noop)
Only copy methods if they don't already exist
Update README.md
Bump version to 0.3.2