Add Table::defaultValues()

This makes it easy to get the default values from a schema table. Having
this is really useful when you want to make an entity with the default
values from the database.

Refs #4398
Fix default values in postgres.

Postgres returns additional data that is not helpful when building
entities or rebuilding schema.

Refs #4398
Merge pull request #4448 from cakephp/schema-default

Add a simpler way to get column values