Extend validation

[Lechus]

Missing func.:
Email validation
isUnique - check db if value exists

[dancryer]

Email validation can be done by using the b8\Form\Element\Email type instead of the b8\Form\Element\Text type.

We couldn't add support for checking the database for a given value, as we wouldn't know what to check. You can implement this yourself using a custom validator. :)