Add authentication to database connection

Hi,

The current connection doesn't support for database authencation. 
In order to use NeoEloquent with a remote DB (ex: Graphene) I needed to be abble to supply the username and password.

> /**
>     * Create a new Neo4j client
>     *
>     * @return Everyman\Neo4j\Client
>     */
>    public function createConnection()
> {
>        $client = new NeoClient($this->getHost(), $this->getPort());
>        $client->getTransport()->setAuth($this->getUsername(), $this->getPassword());
>        return $client;
> }
Merge pull request #25 from thylo/patch-1

Add authentication to database connection