Add a request handler for admin/system

Related to #135, this pull requests adds a request handler for `admin/system` which, among other things, allows you to get the Solr version. The code below is an example of how it can be used:

```php
$query = $client->createSystem();
$response = $client->system($query);
$solr_version = $response->getLuceneSolrImplVersion();
```

On my machine, `$solr_version` evaluates to `3.5.0 1204988 - simon - 2011-11-22 14:54:38`.

The raw data array is a nested mess, so there is a somewhat complex flattening method which I tried to comment as best I could. Sorry about that, but it actually helped me to auto generate the Solarium\QueryType\System\Result class without having to manually write the code pointing to the nested values.