Validate subkey of a XML request

[DavidePastore]

Let's say you have a POST request with a XML in its body:

<person>
   <type>emails</type>
   <objectid>1</objectid>
   <email>
     <id>1</id>
     <enable_mapping>1</enable_mapping>
     <name>rq3r</name>
     <created_at>2016-08-23 13:36:29</created_at>
     <updated_at>2016-08-23 14:36:47</updated_at>
    </email>
</person>

and you want to validate the email.name key. You can do it in this way:

use Respect\Validation\Validator as v;

$app = new \Slim\App();

// Fetch DI Container
$container = $app->getContainer();
$container['apiValidation'] = function () {
  //Create the validators
  $typeValidator = v::alnum()->noWhitespace()->length(3, 5);
  $emailNameValidator = v::alnum()->noWhitespace()->length(1, 2);
  $validators = array(
    'type' => $typeValidator,
    'email' => array(
      'name' => $emailNameValidator,
    ),
  );

  return new \DavidePastore\Slim\Validation\Validation($validators);
};

If you'll have an error, the result would be:

//In your route
$errors = $this->apiValidation->getErrors();

print_r($errors); 
/*
Array
(
    [email.name] => Array
        (
            [0] => "rq3r" must have a length between 1 and 2
        )

)
*/

Things to do:

• implementation;
• add tests;
• improve this PR text;
• improve documentation.

Related to #10.