Consider more advanced validation for 2.0

[ftrotter]

The following rules are too complex for simple implementation using the validation engine, at least in a 1.0 release. But it would be lovely if we could support some of them.

These should be evaluated, and split into specific tickets and then this larger issue closed. Not all of these are good ideas..

It would be nice to have the following, but I do not want to delay too much to get this.

• when there is a something and a something_confirmed, then the 'something_confirmed' rule should be applied to the second field. This one is "between field" and so I am not sure this should be done now.
• when there is a something and a something_different, then 'something:different' should be applied to the second field. This would be helpful for fields like 'cell_phone' and 'cell_phone_different' etc.
• Anything that ends with _file should get the 'file' validation. (not sure how to work with this given the complexities of file storage options on the server-side... )
• Mime type validation for files? using 'mimetypes'
• required_with support?
• support the notion of "sometimes" required using 'sometimes|email' etc