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.

Add compatibility test between releases (resolves issue #103 )

… Yongwoon Cho (#106)

This commit will allows iterative edit and testing to local working copy.
You could iterate edit and testing by the following commands:

 cd /path/to/lz4
 cd test
 make versionstest
 my-favorite-text-editor ../lib/lz4.c
 make versionstest
 my-favorite-text-editor ./test-lz4-versions.py
 make versionstest
 ...

Add supporting iterative edit and testing to versionstest

…rection scenario reported by Takayuki Matsuoka (#110)