Add all datetime fields as nullable columns

To allow nullable values in DateTime fields from Model\User.php,
Propel's schema.xml was updated to match for the generated Propel base
classes. Model\User and Model\UserInterface are updated to default to
null when no value is supplied for setter methods on datetime
properties.
Added credentials_expire_at to nullable columns

Added credentials_expire_at to nullable columns.
BC break - making DateTime props nullable on User

Nullable DateTime fields on User entity
Merge pull request #957 from scribblet/master

Modify user variables to be nullable when expecting a DateTime