Created JsonStrategy for hydrator

## Example:
```php
Class User
{
    protected $roles = array();

   public function setRoles(array $roles)
   {
        $this->roles = $roles;
   }

   public function getRoles()
   {
        return $this->roles;
   }   
}

$user = new User;

$hydrator = new Zend\Stdlib\Hydrator\ClassMethods;
$strategy = new Zend\Stdlib\Hydrator\Strategy\JsonStrategy();
$hydrator->addStrategy('roles', $strategy);
$hydrator->setObjectDecodeType(Zend\Json\Json::TYPE_ARRAY);
$hydrator->hydrate(array('roles' => '["ROLE_SUPER_ADMIN","ROLE_MANAGER"]'), $user);

print_r($user->getRoles());
echo "\n";
print_r($hydrator->extract($user)['roles']);
```

### Output:
```
Array ( [0] => ROLE_SUPER_ADMIN [1] => ROLE_MANAGER )
["ROLE_SUPER_ADMIN","ROLE_MANAGER"]
```

Can these be moved to `__construct` instead? Anything against that?