For #265: Async Client

[matt-boris]

For #265

• Add Async Client (from kinto_http import AsyncClient)
• Add Async Client tests
• Keep 100% coverage
• Update README.rst
• Update CHANGELOG.rst
• Update Makefile to ensure new tests run
• Pull sync Client out of __init__.py file
• Replace unittest with pytest

[matt-boris]

I'm going to replace unittest with pytest. Converting back to draft for now.

[leplatrem]

This is great!

I was thinking that we could have a test that would verify that the same methods are implemented in both clients, so that when we add a new method we don't forget to add it in both...

I left a few comments. If we plan to add support for the batch endpoint later, it would be better to create a follow-up issue to keep track that it's pending.

[leplatrem]

We introduced backoff for the async client. I believe the sync client should obey the same behaviour. I would think that adding decorators there instead of on the async methods would be enough.

[leplatrem]

👍 👍