Add alternative github download URL to PluginManager

The Github downloads are gone

https://github.com/blog/1302-goodbye-uploads

so the PluginManager should be able to fetch zips from alternative location under https://raw.github.com

Plugin authors will have to commit their zip binaries to this different location:

    https://raw.github.com/{user}/{repo}/master/downloads/{pluginname}-{version}.zip

which corresponds to a ${basename}/downloads folder in a Maven project.

Cheers,

Jörg