Issue #94: Update zabbix-vsphere-import to work with Zabbix version 2.4.x as well

* Switch to pyzabbix module from now on as this module is better
  maintained than zabbix_api
* The zabbix-vsphere-import tool now works with all versions supported by
  pyzabbix, which at this moment is Zabbix versions from 1.8.x to 2.4.x