Use decompressobj for zlib decompressing, solving issue #224 and #238

With `decompressobj`, some `.cmt.xml` files which could not be decompressed can now be.

The issue #224 and #238 can only be experienced from time to time so that it is hard to reproduce. I have made tests with both old and new code **at the same time with the same URL**. So I am sure that `decompressobj` solves this problem.