Add Middleware feature, move validation to middleware

[nathan-fiscaletti]

Add a middleware feature. Validation.php would be removed, and a Middleware interface would be created. Validation would be moved to a custom implementation of this Middleware, and API Key validation would be moved to a separate Middleware implementation. The regular request validation Middleware will be enforced, but API Key validation will be optional.

Instead of using the $is_public, $white_list_keys, and $security_level properties of the RequestHandler, this would be moved to custom Middleware implementations.

To add middleware to an Endpoint, you would override the $middleware property and set it to an array of the Middleware classes that would be used.

You can create custom middleware by running ./synful -cmw MiddleWareName from console. You would then override the action function of the Middleware object. This will give you access to the Request object.

interface MiddleWare
{
    public function performAction(Request $request);
}

[nathan-fiscaletti]

Completed in PR #88