Various updates to zproto server generator

- dependency on CZMQ shifted to 2.2.0
- now allows pipelined requests (queues until hits external state)
- many smaller improvements, needed by zpipes server
Added const qualifiers to strings
Set ABI version to 0:0:0
bind method now accepts format specifier
Generate BNF terms selectively
Add group types, fixed #31
Fixed compile error in msg parsing

- when two messages used msg fields with same name, caused duplicate
  variables to be generated

- affected Zyre

- simplified code so no variables are needed in this case
Merge pull request #29 from hintjens/master

Various updates to zproto server generator