Problem: Makefile.am needs 'code' target for models

This lets us regenerate code that's produced from models via GSL,
which we use for the zgossip_msg codec and the zgossip server.

Solution: add <model name = "xxx" /> to project.xml and treat
this in the code generator template for src/Makefile.am.
Added first draft of zgossip class

- also moved XML models into src so that "make code" works on them
- zgossip_msg.xml defines messages for gossip protocol
- zgossip.xml defines the state machine for the gossip server
- a rough explanation of the gossip model is in zgossip.c
- next step: a fuller test that explores all edge cases
Merge pull request #507 from hintjens/master

First draft of zgossip class