Support rails accepts_nested_attributes_for

Shout out to @kevinthomson for figuring this out first ([blog post](kevinthompson.info/blog/2012/12/12/supporting-rails-nested-attributes-in-batman-js.html), [gist](https://gist.github.com/kevinthompson/4219217)). I just tweaked it a bit. It's about time!

Todo:

- [x] test polymorphic associations

Anything else?