KnpLabs/php-github-api#178 - adds a basic search api implementation.

Example usage:
````php
$client->api('search')->find('issues', "repo:$repo $filter");
````

The new search api seems to need its own class since it returns objects in a different format. This basic function seems to mimic your API syntax pretty closely.
Instead of this, it would be much more consistent to have 4 methods on the class