If no method attribute on root, check for data-method

[CaptainN]

To keep inline with standards, the root element shouldn't necessarily have a "method" attribute, if it isn't a form element. OnceForm does need to know which method to use, so it must be configurable. This can be either API level, though that would require a user to manually recreate init - which would suck.

We could also use the data attribute standards, and just allow for the use of data-method instead of method.