Fix SnappyFramedDecoder issues

- Checksum header was being incorrectly read due to incorrect order of
  shift and masking operations.
- Length field of 1-byte copy was being incorrectly interpreted due to a
  typo in the binary mask used to extract it.
- Use ByteBuf.readUnsignedByte() instead of readByte() & 0xff
- Use bitwise-OR wherever possible
- Use EmbeddedByteChannel to test
- Use ByteBuf comparison instead of array comparison
- Work done by @lw346 and then revised by @trustin