Feature/rewrite for client side eval

[matthewelwell]

This PR contains a full rewrite of the Python client to allow for client side evaluation. As part of this work, I have also added the possibility for providing a list of default flags when initiating the client.

Some general design decisions:

1. The client only exposes 2 methods, one to get an environments flags and another to get the flags for an identity (which also can optionally persist a list of traits).
2. The client returns a Flags class, from which, users can access specific flags and their state / value. This means that users will have to write a little more code to get the value of a given flag, however, it will only make a single API request to do so (for users not using client side evaluation).

Options for extension:

1. Caching
2. At the moment, defaults are only used when the API request is successful but the feature doesn't exist. We should extend this to allow the defaults to be used when the API requests are failing.