Added ability to override class name for uniqueness validator

Closes #519

Added `:client_validations` option to the `validate_uniqueness_of`,
where one can specify the real class name for the remote validation.

For example, we have a wrapper class for the User model, UserForm, and
it uses remote uniqueness validation for the email field.

```
class UserForm
  include ActiveRecord::Validations
  attr_accessor :email
  validates_uniqueness_of :email
end
```

However, this won't work since middleware will try to perform validation
against UserForm, and it's not persisted.

Now one can state which class should be used for validation:

```
class UserForm
  include ActiveRecord::Validations
  attr_accessor :email
  validates_uniqueness_of :email, :client_validations => { :class =>
  User }
end
```