Key-Value storage

- [ ] Code
- [ ] Test
- [ ] Documetation

```php
<?php

/**
 * Basic usages
 */
at_kv($bin)->set($key, $value, $ttl);
at_kv($bin)->get($key);
at_kv($bin)->set(array($key_1 => $val_1, $key_2 => $val_2));
at_kv($bin)->get($key_1);
at_kv($bin)->get($key_2);

/**
 * With custom engines
 *
 * ------------------------------------------------------
 *     Override
 *   ------------------
 *
 *  To override default engine/otpions for specific $bin, in settings.php
 *  at code similiar to these lines:
 *
 *  @file settings.php
 *
 *  $at['kv'][$bin]['engine'] = 'db';
 *  $at['kv'][$bin]['options'] = array(
 *    'ttl' => '+ 2 hours',
 *    'database' => array(
 *      'source' => 'default',
 *      'target' => 'write_db',
 *    ),
 *    'table' => 'table_with_fast_backend',
 *  );
 *
 *   ------------------ 
 *      Extends
 *   ------------------ 
 *
 *  Define new service, tagged it with 'kv', the class has to implement
 *  \Drupal\at_base\Helper\Key_Value_Interface.
 * 
 * ------------------------------------------------------
 */

# ---------------------------------------------------------------
# Engine => Database (default engine)
#   Default Table: at_kv
# ---------------------------------------------------------------
at_kv(
    $bin, 
    $engine = 'db',
    $options = array(
      'database' => array(
        'source' => 'default',
        'target' => 'default',
      ), // which configured values from $databases['default']['default']
      'table' => 'at_kv'
    )
  )
  ->set($key, $value);

# ---------------------------------------------------------------
# Engine => Disk
#  Default directory: %private/kv
# ---------------------------------------------------------------
at_kv($bin, $engine = 'disk', $options = array(
  'dir' => '%private/kv',
))->set($key, $value);

# ---------------------------------------------------------------
# Engine => APC
#  Default prefix: substr(md5(conf_path()), 0, 7)
# ---------------------------------------------------------------
at_kv($bin, $engine = 'apc', array(
  'prefix' => substr(md5(conf_path()), 0, 7),
))->set($key, $value);

# ---------------------------------------------------------------
# Engine => Memcache
#   Default prefix: substr(md5(conf_path()), 0, 7)
# ---------------------------------------------------------------
at_kv($bin, $engine = 'memcache', array(
  'prefix' => substr(md5(conf_path()), 0, 7),
))->set($key, $value);
```