Allow Configurable Separator in JsonRpcMultiServer

We have existing infrastructure where the JSON-RPC method names are of the form `Service/method` and I would love to use your project yet maintain compatibility with existing services in your environment. Would you consider allowing the service-method separator to be configurable? Thanks!
Merge pull request #39 from gresrun/patch-1

Allow Configurable Separator in JsonRpcMultiServer