Llamada AJAX POST enviando json y recibiendo json en el response con Slim PHP

api.php

<?php 

//	header('Cache-Control: no-cache, must-revalidate');
//	header('Expires: Mon, 26 Jul 1997 05:00:00 GMT');
//	header('Content-type: application/json');
	
	define('PETICION_OK'						,	200);
	define('PETICION_NO_AUTORIZADO'	,	401);	
	
	//Login
	$app->post('/api/login', function () use ($app)
	{
		
		$username = $app->request()->post('user');
		$password = $app->request()->post('password');
		
		$usuario = Model::factory('User')->where('id',$username)->where('psswd_md5',$password)->find_one();
		
		$body = NULL;
		$code = PETICION_OK;
		
		if($usuario)
		{
			$profile = Model::factory('UserProfile')->where('user_id',$username)->find_many();
			$body =  array(
				'msg' => 'Welcome '.$usuario->name,
				'user' => $usuario->as_array(),
				'profile' => $profile->as_array(),		
			);		
		}
		else 
		{
			$code = PETICION_NO_AUTORIZADO;
			$body =  array(
				'msg' => 'Invalid user or password',
			);	
		}
		
		$app->outputJSON($body,$code);
	});
 ?>

ccslim.php

<?php 

	class CCSlim extends Slim\Slim 
	{
    function outputJSON($data,$status) 
    {
        switch($this->request->headers->get('Accept')) 
        {
            case 'application/json':
            default:
                $this->response->headers->set('Content-Type', 'application/json');
                $this->response->status($status);
                echo json_encode($data);        
        }       
    } 
}


 ?>

index.php

// En el fichero index.php del proyecto Slim:

// Prepare app
$app = new CCSlim(array(
    'templates.path' => '../templates',
));
/*
$app = new \Slim\Slim(array(
    'templates.path' => '../templates',
));*/

post.js

  $('#btnSignIn').click( function() 
	{ 
		dismissMessage();
		$('#btnSignIn').attr("disabled",true);
		var usuario = $.trim($('#user').val());
		var pass = MD5($.trim($('#password').val()));
		
		var call = $.post('/api/login', {user:usuario, password:pass});
		

		call.done(function(response) 
		{
			storeCredentials(response);
		});
		
		call.fail(function(response) 
		{
			showMessage(response.statusText);
			$('#btnSignIn').attr("disabled",false);
			
		});
		call.always(function() 
		{
		    console.log('Finished');
		});
	
	});