Improved and documented proxy support

While it was always possible to use the development environment behind a proxy, you had to know about the vagrant proxyconf plugin; the process wasn’t part of the documentation and some post install manual setup was still required to get Google Chrome to use the proxy.

This has now been addressed by updates to both the Getting Started documentation and the Configuration documentation; the vagrant-proxyconf plugin is now auto-installed and the GNOME proxy is configured as part of the provisioning; the proxy configuration is part of the standard Nugrant configuration approach, so you can set the default values in the Vagrantfile and users can override it in their .vagrantuser files if necessary.