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.
That str_replace makes a BC break, please remove that for the 2.5.3