…rror` + Fix #65 (#87)

# Summary
Extended typing, adjusted doc strings, added a pre-commit hook for mypy

**Breaking**
- The "EGeneral:Unknown method" response will now raise the
`KrakenException.KrakenUnknownMethodError`
- "EAPI:Bad request" will raise `KrakenException.KrakenBadRequestError`

**Bug fixes**
The broken Spot endpoints for create_order_batch and cancel_order_batch
were fixed. Closes #65

…pair` parameter optional (#93)

# Summary

As mentioned in #90 - the /AssetPairs endpoint do not require a `pair`
query parameter. Since this parameter is now optional, the function name
did not match, so it was renamed from `get_tradeable_asset_pair` to
`get_asset_pairs`.

…balance` (#98)