Create an implementation of FediverseNode that acts as a JSON-RPC client

Replaces #95. Tried this out, and it's so much simpler. To integrate a new application, all you need to do is put a new handler into the URL namespace of your application (say `example.com/_feditest_json_rpc.php`) and dispatch on the `method` field in the JSON posted to that endpoint. Other than JSON parsing, no libraries required.