LZ4 support

Since komiya-atsushi/lz4-ruby#3 and komiya-atsushi/lz4-ruby#7 are now fixed, and v0.3.2 is out, cql-rb can finally get LZ4 support.

Turns out that Cassandra also uses a non-standard LZ4 format, the encoded data is prefixed with an int with the uncompressed size. Very convenient, but also completely undocumented as far as I can tell.