Callosum

* MAINTENANCE: Update dependencies and only specify the minimum versions since
Callosum is a library.

* FIX: wrong typing of `RPCMessage.body` field

* IMPROVE: Add `debug_rpc` option to `rpc.Peer` for logging exceptions in RPC
scheduler and user-defined handlers explicitly.

* Update dependencies and remove unused ones.

* First public release with a working RPC based on ZeroMQ DEALER/ROUTER sockets.


2018-05-02

* Started the project.