Created plugins.xml

To move away from virtu.al to Github to make plugin updates a little more useful (or at all).

This probably needs centralising at some point so that all vCheck projects use the one update source.
Update vCheckUtils.ps1

Added Get-vCheckPluginXML to return plugin.xml to make plugin updates easier.

Moved plugin.xml URL to Github, as well as the plugin source to use Github. Should be no more ties to virtu.al domain now, so updates can be performed by anyone and submitted via pull request.
Merge pull request #127 from Sneddo/pluginupdates

Updates to plugin handling