COMPRESS-272 add auto-detect for .Z

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/compress/trunk@1586869 13f79535-47bb-0310-9956-ffa450edef68
COMPRESS-272 document the fact .Z hasn't been auto-detected so far

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/compress/trunk@1586873 13f79535-47bb-0310-9956-ffa450edef68
COMPRESS-277 make sure IOUtils#skip deals with streams that return 0 on skip even though more could be read

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/compress/trunk@1586879 13f79535-47bb-0310-9956-ffa450edef68