Pass session data in meta back in reply.

This session data is sent back to help identify the originating message
in stateless systems.
Better JSON stringify errors in the reason.

Now errors are `toString`'d, but other types (arrays, objects, etc) are
left as their JSON-equivalent type.
Pass meta to rpcReceive callbacks.

The meta can include useful information identifying the requester.
Bump version to 0.5.0.
Merge pull request #28 from nubs/master

Pass session data back in RPC responses and fix non `Error` instance failures in replies.