First cut of json-rpc 2.0. 

It is still not tested well. But please comment on the changes.
I have added 2 classes to rpc directory. I think they will be useful for other codecs also. 
I also made some changes to rpc/server.go to accomodate json-rpc 2.0
Please take a look and tell me if design is okay.
I'll refine the code/add comments and more tests later.