Implement a function to create stable tarballs

[mojca (Mojca Miklavec)]

Many components in the core need a function that is able to create a stable tarball:

• creating tarballs from SVN checkouts, potentially from git repositories with submodules etc.
• creating reproducible tarballs with binary builds

Options:

• folder whose contents have to be compressed
• compression method / extension
• name of the prefix (usually that would be ${distname})
• (optional?) timestamp of the resulting tarball
• (optional?) whether or not to change timestamps of all files (svn checkout might already have suitable timestamps when use-commit-times=yes is set)