add a test for deep form nesting and validation errors
allow unlimited nesting of forms by making #validate recursive. this fixes #68.
introduce Errors#valid? that keeps track of sanity.
remove Forms#validate_items as this is done now in #validate: #validate! is called on every collection item automatically.
move #validate and friends into validate.rb.
remove more methods from Forms as this is all implemented with the recursive #validate!.