Proposed fix for Issue 39 - Append "api_jsonrpc.php" to $this->zabbix_url in Zabbix class ctor

A minor change to address automatic login failure by appending "api_jsonrpc.php" to $this->zabbix_url in the Zabbix class constructor, this also makes it consistent with setting of similar URL variables in the constructor ($this->zabbix_url_chart and $this->zabbix_url_index)

See also: https://github.com/mattiasgeniar/MoZBX/issues/39