Removed web2py logic for default validators

[gi0baro]

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.

[mdipierro]

would this break anything?

[gi0baro]

Let me merge this, I'm updating the PR on web2py to use this.

[gi0baro]

This is covered by: gi0baro/web2py@dd73678

So the PR web2py/web2py#561 is still valid.