Kraken API Client: Public Info

[echavemann]

Create a pysrc/adapters subdir to implement a requests handler for the public API (no key required) for kraken.

The recommended directory structure is as follows:
adapters/kraken/containers.py: Python objects (with strict typing) to encapsulate our responses
adapters/kraken/client.py: Requests client itself. Inlcude reasonable method names. No need to figure out authentication just yet, though you will need to include both the URL for testnet and live, and switch using a testnet parameter passed into clients when instantiated.

Testing:
Integration tests are recommended for the testnet functions, as well as unit tests for complicated packing into response objects. Note that you should use the generic SnapshotMessages and trade messages if it is reasonable.

Docs:
https://docs.kraken.com/api/docs/category/rest-api/market-data