use "tar xf" for compression autodetection

GNU tar 1.15 (2004) and later supports auto-detection of compression.
GHC 7.8 started shipping .tar.xz binary tarballs in addition to .tar.bz2,
so this change should allow using either with hptool and also .gz, etc.
Merge pull request #107 from juhp/patch-1

use "tar xf" for compression autodetection