Implemented ZMTP/2.0. Introduced ZMTP10Codec and ZMTP20Codec

* Implemented the ZMTP/2.0 handshake and framing mechanism
* Moved handshake mechanism into ZTMP10Codec and ZMTP20Codec
* Made ZMTP*Codec dynamically update their pipeline with the appropriate
  ZMTPFramingDecoder instances
* Updated documentation and test cases