Zstandard v0.6.1

New : zlib wrapper API, thanks to @inikep
New : Ability to compile compressor / decompressor separately
Changed : new lib directory structure
Fixed : Legacy codec v0.5 compatible with dictionary decompression
Fixed : Decoder corruption error (#173)
Fixed : null-string roundtrip (#176)
New : benchmark mode can select directory as input
Experimental : midipix support, VMS support