Issue #103 : Add compatibility test between releases

Issue #103 : Add compatibility test between releases

Add forward/backward compatibility test between all versions

 - Add new Makefile target "versionstest".
 - Standard test procedure like "make versionstest && make clean"
   works properly.

test/test-lz4-versions.py

 - This script creates test/lz4test and checkout all
   Release tagged versions.  After that, compile all versions of
   lz4c and lz4c32 and compress/decompress between all versions.

 - This test clones entire git repository to checkout all tags.
    - Since Travis's default command does not clone entire repository,
      we could not checkout all tags from Travis' default clone.

 - After that, test script shows all unique .lz4 files.  In this list,
   'r999' means 'head' of current working copy.