Implement non positional parameters as an option of the client.

As positional is optional ( see http://www.jsonrpc.org/specification ).
Usage :
Jimson::Client.new(url,opts = {}, namespace = nil, positional = true )

As you can see positional is the default value in order to preserve the original Jimson behaviour. To use non positional parameters Initialize the client with positional = false

Still to be done :
tests/specs
Server implementation