Add support for github version downloads
EDIT: If the code of this MR is significant enough to be subjected to copyright, I'm licensing it under the MIT, since I'm planning on using some of this code in another MR for another MIT-licensed project. There shouldn't be any problem in including MIT code in a GPL licensed project, while it can be quite problematic to include GPL code in an MIT project, that's why I'm doing this.
This merge request adds support for downloads from github. This only works for stable releases above (and including) 3.1.1, since Github only knows about stable releases, and v3.0.* and v3.1 all have no assets for some reason (might be because Github only stores the assets of the latest versions and therefore the older versions have no assets, if that's the case, the code needs to be rewritten to take into account the release of new versions).
This was requested in #9.