Implement SMD (Service Mapping Description)

It would be great if jsonrpc would be able to list used server options and provided methods according to Service Mapping Description protocol [1], because some JSON-RPC clients require it to work properly [2](currently I forced to create SMD file by myself).

[1] https://github.com/dojo/docs/blob/master/dojox/rpc/smd.rst
[2] jQuery JSON-RPC Server - http://www.tanabi.com/projects/jsonrpc
