JSON-RPC 2.0 compliance

[chungers]

This small PR switches the Codec from an outdated JSON-RPC 1.0 implementation to the latest 2.0 spec implementation.

This impacts those who want to develop plugins using a different language / tooling for JSON-RPC. A short summary of over the wire representation can be found here: https://en.wikipedia.org/wiki/JSON-RPC#Version_2.0

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "json-rpc2" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354236520
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[wfarner]

LGTM once the PR checks pass. Thanks for investigating this!