wip: netty4

This is work in progress. 

Tests pass, but it might be nice to clean up some dusty corners (ZMTPSession, I'm looking at you) while we're breaking things anyway.

@noaresare Wondering what the best way to structure this is. Netty 3 and Netty 4+ can (and might in some cases have to) coexist in the same project, so it would be nice to have netty-zmtp for 3 and 4 be able to coexist as well. One way forward would be to split netty-zmtp up into _netty3_, _netty4_ and _common_ modules, keeping all the code in the same project and repo. Thoughts?