Aiosnow

- Add support for the Attachment API, either directly or via a TableModel
- Add TableModel tests
- Improve test fixtures
- Allow passing of return_only to TableModel.get
- Rework *models* package structure
- Clean up unused exceptions
- Remove support for passing of custom aiohttp.session objects to Client
- Rework the reference documentation

Note about Client *session* parameter removal:
Controlling what the `Client` session factory produces by passing a *session* to its constructor is no longer supported. Instead, the `Client.get_session` method should be overridden to accomplish the same thing.

Reworks the `aiosnow.Client` and its `aiohttp.ClientSession` factory.

This release addresses a number of issues related to Model Schema nesting:

Highlights:
- Improve schema registration
- Add support for any level of nesting
- Fix issue with cached documents not getting accessed correctly in the expansion code

- Chained conditions are now stored in a local registry
- QueryBuilder renamed to Selector
- Selector interface improvements

- Fix Condition serialization via `__str__`
- Separate querying methods from fields. Adds new type <Field>Queryable: a mixin that
can be used with `BaseField` to make fields queryable
- Fix issue with setting primary key in `BaseModel` in some cases
- Refactor the query package

- Decouple Client and Model
- Couple Model and Schema
- Improve request error handling
- Mapped fields refactoring: Mapping.id => Mapping.key
- Remove Choice field types
- Remove Email field type
- TableModel.get_one() now returns a Response object rather than a dict
- Fix IntegerOperator LE/GE bug
- Fix issue with subclasses with common superclass getting attributes overridden