Updated CodecFactory to reference constants for codec names
Fixed a small bug in Snappy and BZip2: compression/decompression were incorrect if the input ByteBuffer was a a slice(). I don't see anywhere that this would actually happen currently, but some codecs were written to account for this correctly; now they're all correct. Updated everything to compute the correct offset into the underlying array. (I can add a test for this in TestAllCodecs once #351 is merged).