pyplugin_installer moved to QNetworkAccessManager from QHttp

As discussed in previous pull request https://github.com/qgis/Quantum-GIS/pull/633

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.

James