Add builtin _zerorpc_ping method

The protocol doc mentions a _zerorpc_ping method. This implements
the _zerorpc_ping method with a return value compatible with the
Python implementation.
Bump msgpack dependency to 0.2.4

The 0.2.x release of msgpack adds toJSON compatibility to control
how objects are serialized. This is particularly useful if you want
to hide some properties (or object functions) from being serialized
and sent through ZeroRPC.
Merge remote-tracking branch 'origin/builtin-ping'
Merge remote-tracking branch 'origin/msgpack-0.2.4'