Move plugin_installer from QHttp to QNetworkAccessManager

I have done some fairly simple modifications to the plugin_installer plugin so that it now uses QNetworkAccessManager instead of QHttp. QHttp is apprently deprecated and also limits plugin repos to be hosted on HTTP servers. Using NAM allows repos to be hosted in more varied locations.

I think there is an outstanding problem with the status updates because NAM and QNetworkReply do not report the same things as QHttp used to. However the functionality works.

Please be gentle... my first ever pull request/FOSS contribution

Regards,
James