Allow MongoClient options to be passed from config array

The second parameter to MongoClient::__construct is an array of $options, including connecting to replica sets and setting read preferences:

http://php.net/manual/en/mongoclient.construct.php

At the moment zf-oauth is automatically passing a simple array with one option as value of this paramater:

```
$mongo = new MongoClient($server, array('connect' => false));
```

This pull request allows for setting $options in the configuration while preserving the default behavior of leaving 'connect' as false:

```
$options = isset($config['zf-oauth2']['mongo']['options']) ? $config['zf-oauth2']['mongo']['options'] : array();
$options['connect'] = false;
$server  = isset($config['zf-oauth2']['mongo']['dsn']) ? $config['zf-oauth2']['mongo']['dsn'] : null;
$mongo   = new MongoClient($server, $options);
```