Rewrite client for ThreatExchange API from scratch

[jiefugong]

Summary

This work is being done during the Initiate Hackathon - a lot of changes are still needed!

Rewrites the client for ThreatExchange's public API from scratch. In this PR, we have the following:

1. Adds a core.py file which contains the implementation for the client
2. Adds a models.py file which contains some definitions of objects that one can interact with in the API

The models.py file is NOT fully vetted for correctness. As each API implementation is filled out, we will modify / create and verify the models that are associated with that API endpoint. In this PR, we implement a basic endpoint for demonstration (get_members())

Test Plan

Follow the README.md - set up a virtual environment and download required packages. Set up local access token for RF Test application. Afterwards, you can run the following:

from fb_threatexchange import ThreatExchangeClient

client = ThreatExchangeClient()
members = client.get_members()

[Dcallies]

This library is written as though it is going to be a standalone package. Currently the threatexchange client is distributed as part of threatexchange.

Can you talk through why you think distributing it is a standalone package is preferred?