Add code to eventually support .xz compressed tarballs
We switched from providing .tar.bz2 files on CollecTor to .tar.xz around July or August 2014, but we never updated metrics-lib to parse those.
We should try out how slow parsing .xz compressed tarballs is compared to uncompressed tarballs. Maybe the better plan is to put out a big warning and suggest that users decompress themselves using the
When we add support for this, let's try not to add any new dependencies to metrics-lib.