Memcache Session Adapter

I would like to contribute this Memcache session adapter for the incubator.

Here a short overview how it works:

```php

$di->set('session', function(){

    $memcache = new Phalcon\Session\Adapter\Memcache(array(
        'host'          => '127.0.0.1',     // mandatory
        'post'          => 11211,           // optional (standard: 11211)
        'lifetime'      => 8600,            // optional (standard: 8600)
        'prefix'        => 'my-app'         // optional (standard: [empty_string]), means memcache key is my-app_31231jkfsdfdsfds3
        'persistent'    => false            // optional (standard: false)
    ));
    $memcache->start();
    return $memcache;
});

```