Upload scoreset

[joemin]

Another long(ish) one, but probably easier to read because it's split across many more files.
Also, the auth_token included in the example files are for a local instance of Mave so it's fine that it's hanging out in clear text.

Changes:

• Client: a client that handles HTTP requests to a running instance of MaveDB (either deployed or local)
• Models: a set of model objects that mirror the model objects that already exist server-side
• Examples: an examples directory that demonstrates how to upload/download experiments and scoresets
• Test Files: some small, basic files to test scoreset_upload

Implementation Notes:

• I wanted the client to be agnostic of the actual models that it was serializing/deserializing from/to so all of that logic is wrapped up in the model classes.
• Had to use attr.s instead of built-in data classes because when deserializing from JSON to instances of the models, keyword order isn't guaranteed unless we enforce it, and the data classes only take *args in their inits instead of **kwargs, which is super annoying. So, instead it's attr.s where every property has kw_only=True but... what's one to do, right?