Add LZ4 compression support

Broken because lz4-ruby doesn't handle multi-byte characters
Update to lz4-ruby 0.3.2

This one supports multibyte characters
Generalize the compression integration tests and add one for LZ4
Update the docs to say that lz4-ruby 0.3.2 is required
Add info about LZ4 to the readme

[ci skip]
Optimize the LZ4 compressor by constanticizing strings

It's not a big optimization, but it's something