Memcache adapter

Cache storage adapter for [ext/memcache](http://us3.php.net/manual/en/class.memcache.php) (not to be confused with existing ext/memcached adapter).

Implements feature request: https://github.com/zendframework/zf2/issues/4481

An example configuration for `Zend\Cache\StorageFactory` would be:
```php
$memcacheStorageAdapter = \Zend\Cache\StorageFactory::factory(array(
    'adapter' => array(
        'name' => 'memcache',
        'options' => array(
            'compression' => false, // use compression on all writes when able?
            'servers' => array(
                0 => array('host' => '10.0.0.20', 'port' => 11211), // shorthand, server_defaults will be applied
                1 => array('host' => '10.0.0.21', 'port' => 11211, 'status' => true), 
                2 => array('host' => '10.0.0.22', 'port' => 11211, 'status' => true, 'weight' => 3, 
                           'persistent' => false, 'timeout' => 2, 'retry_interval' => 20), // or can specify all values
            ),
            'server_defaults' => array( 
                // defaults that are applied to servers with missing values
                'weight' => 1,
                'persistent' => true,
                'timeout' => 1,
                'retry_interval' => 15,
            ),
            'lib_options' => array(
                'compress_threshold' => array('threshold' => 100, 'min_savings' => 0.2),
            ),
        ),
    ),
));
```