Issue 18: implement JSON-RPC API

Created a simple python JSON-RPC server script called ngccc-server.py
that allows you to call all the same functions as in ngccc.py but from
a JSON-RPC API.

To start the server run:

python ngccc-server.py localhost 8080

After that, you can connect to localhost:8080 and do JSON-RPC calls.

Note that pyjsonrpc is now required.
Merge pull request #19 from jimmysong/18

Issue 18: implement JSON-RPC API