Add A XZ Codec

The java Avro projects already depend on the commons-library; the attached patch adds support for the XZ codec using classes from it. I've also refactored some of the duplicated code in the tools project (the code that deals with the --codec and --level options) by moving a single copy to the Util class, and calling that from the various tools that produce Avro files.

The XZ codec by default uses the LZMA2 algorithm to compress files - this produces noticeably smaller files than the current bzip2 or deflate codecs.