Added methods to use Sistrix API

Fixes issue #88.

Attention: Changed output of getVisibilityIndex() to a real integer (contained a comma as a decimal point before) like documented and in consistency with data returned from the Sistrix API. Depending on the usage of this method, this may break some installations so we should include a warning somewhere.
I'm not sure to move this in a separate method or if that has a entitlement to be here.

If i look strictly I see a site effect, I give your the chose.