zstd: 0.5.0 -> 0.5.1

Changes:
- New: Optimal parsing => Very high compression modes, thanks to @inikep
- Changed: Dictionary builder integrated into libzstd and zstd cli
- Changed (!): zstd cli now uses "multiple input files" as default mode. See zstd -h.
- Fix: high compression modes for big-endian platforms (#123)
- New: zstd cli : -t | --test command