LZ4 partial library source

The following files were pulled from: 
    http://github.com/Cyan4973/lz4/tree/master/
at commit dfabec2d18

lib/lz4.c
lib/lz4.h
lib/lz4hc.c
lib/lz4hc.h

The remaining files are unnecessary since we don't use the lz4 framing format.

