Update README to account for PHP's stupidity

See http://stackoverflow.com/a/9095022/323767 for why.
Fix error in validation.

Allows either multiple validations to be in an array, or a single
validation as the value the field.

$validation = array(
    'name' => array(
        Validation::presence()
    )
);

vs

$validation = array(
    'name' => Validation::presence()
);