Python based add-on installers

This adds support for python based add-on installers.

The installers are called as vfs plugins with parameters, e.g. action=install&package=foo.bar&version=1.0.3

this is all or nothing; if a repository comes with a .py installer, the installer need to take care of everything, including downloading and dependency resolving.

this allows for use of a system's package manager to install add-ons, e.g. apt or yum.