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
simplified updatePipeline(), skipping the remove/re-add of elements
factor out parseZMTPHeader switch
fix for botched search/replace FOO -> identity
Use ZMTPSession as a config object for ZMTP*Codec

* Introduce ZMTPSocketType session property
* Update tests to new ZMTP*Codec signature
Got rid of star imports
ZMTPSession version cleanup

* Got rid of some ZMTPFraming{Decoder|Encoder} constructor parameters as
  per the review discussion
* Renamed ZMTPSession protocolVersion property actualVersion and clarified
  it's semantics in the API doc.
Test case fixes for the ZMTPFramingEncoder signature change
Merge pull request #7 from noaresare/pr/zmtp2

Implemented ZMTP/2.0