Add the option to set authentication and ignore SSL for Graphite connection

I've added two configuration parameters: `graphite_auth` (default: `nil` - no authentication) and `graphite_verify_ssl` (default: `true`, verify certificate). This two are used to better control the GET request to Graphite (in our case we use basic auth & problematic SSL certificate).

I've added specs for the configuration, but seen none for the graphite GET call so haven't added those.