Updated examples

The examples were outdated with respect to the new adapter requirement
Added section on using an Select object

There was no previous mention in the docs of the possibility to use a
Select object to seed the validator. Added with example.
Removed section on database adapters

Since this section doesn't describe how to actually retrieve a database
adapter, it adds nothing over the previous examples.
If there is need for an example of how to get the adapter, this section
should be re-added.