Add MongoDB adapter for Zend\Cache

Adds a MongoDB adapter for Zend\Cache, along with its own Options class.

Usage is the same as any other adapter - just throw an array of config at StorageFactory::factory() and it will handle the rest.
```php
$config = array(
    'adapter' => array(
        'name'    => 'mongodb',
        'options' => array(
            'libOptions' => array(
                'server' => '',
                'database' => '',
                'collection' => '',
                'options' => array(),
                'driverOptions' => array(),
            ),
            'namespaceSeparator' => '',
            'resourceId' => ''
        ),
    ),
);
```
The libOptions for collection, server and database are required. server will most likely look something like `mongodb://localhost`. database and collection should be used to define the name of the MongoDB database and collection to use for the cache (Mongo will create these on first use if they do not exist).
libOptions->options is passed to Mongo|MongoClient's __construct as the options parameter - so you can set fsync, write concern, username, password etc here. libOptions->driverOptions is also passed as the third parameter - these are generally used for connection context settings or logging callbacks. See http://www.php.net/manual/en/mongoclient.construct.php for all options

Tests are included following the example of the existing adapter test cases