json rpc 2.0 named parameters support + error code support

Hi Gimmy,

I am using your jsonrpcjs, it is great and simple. They are two things, they are missing in this code.
1) absence of support of named parameters, in your version they are passed using order
2) possibility to work with the code of json rpc error.

I am succesfully using  your jsonrpcjs extended with above 2 mentioned features.
I created fork and I would like to ask you to merge these fork to your trunk in the next version of jsonrpcjs, if possible. I guess it could be helpful for many users.

Kind regads,
Michal