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(
            'collection' => '',
            'connectString' => '',
            'database' => '',
        ),
    ),
);
```
The options for collection, connectString and database are required. connectString 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).

Tests are included following the example of the existing adapter test cases