Removed web2py logic for default validators

`sqlhtml_validators` method in helpers was based on web2py's validators logic.

This commit update the behavior of default validators addition to fields:
* now the addition is made by the `auto_validators` method
* validation addition can be used with the `DAL.validators` attribute, which should be a `dict` in the form `{'field_type': validator}`
* otherwise addition can be performed by a custom method defined into `DAL.validators_method` attribute.