Added Zend Cache Adapter

Added a Zend Cache Adapter so one can reuse the applications configured cache.  This should open up a whole range of caches like Zend Server, Memcached, etc.

To use you must have a configured service for Zend Cache.   Please see the [docs](http://framework.zend.com/manual/2.0/en/modules/zend.cache.storage.adapter.html) for a detailed walk through on how to setup Zend Cache.

Sample module config:
```php
<?php
return array(
    'asset_manager' => array(
        'caching' => array(
            'default' => array(
                'cache'     => 'ZendCache',
                'options' => array(
                    'service' => 'myAppCache'
                ),
            ),
        ),
    ),
);
```

A sample configuration for Zend Cache using Memcached as a back end might go as follows:

File: module/Application/Module.php
```php
<?php

namespace Application;

class Module
{
    public function getServiceConfig()
    {
        return array(
            'factories' => array(
                'myAppCache' => function ($serviceLocator) {
                    $config = array(
                        'adapter' => array(
                            'name' => 'Memcached',
                            'options' => array(
                                'servers' => array(
                                    array('localhost','11211'),
                                ),
                                'namespace' => 'zfcache',
                            ),
                        ),
                        'plugins' => array(),
                    );
                        
                    return StorageFactory::factory($config);
                },
            ),
        );
    }
}
```