ContestImporter: support .xz compressed tarballs.

When we'll start using Python >= 3.4, tarfile will have native support.
So far we have to manually build the decompression pipeline.

XZ compression during exporting isn't supported so far.

This features requires package lzma, which has entered the Python
standard library only in 3.3. Now one has to manuaaly install the
lzma package, though I don't think this feature is important enough
to deserve that lzma is added among the requirements.