Added LZ4Compressor.

Added [LZ4](https://code.google.com/p/lz4/) compression to be used with the new LZ4Compressor in place of the Compressor or the GzipCompressor. To use, just install the [lz4-ruby](https://github.com/komiya-atsushi/lz4-ruby) gem.

Benchmarks are included and show an order of magnitude compression/decompression speed improvement while increasing compressed data size by about 1/3.