array attribute

Issue #53

I hacked in a special case for array attributes that addresses the `isDirty` problem by creating an array proxy, observing it, and calling setProperty with a copy when it changes. It seems to work, although it is not the cleanest thing in the whole world.

The array proxy is used automatically for attributes with '[]' in their name. You still have to define a custom transform.

Please review. This is an important issue.