Minor Updates

See PR #22:

Coalesced record request methods where possible. And:

Addressed Issue #16

• Airtable() instances can now be created inside and outside async with blocks:

at = Airtable()
....
at.close()
# vs
async with Airtable() as at:
....

Addressed Issue #17

• post_record, post_records, updated_record and update_records methods return created or modified records.
• delete_record and delete_records return ids and deleted status of records.
• errors return response data payload.

Addressed Issue #18

• Response codes 500 and 502 added to exponential backoff

Fixed Issue #19

• Added logging_level kwarg to all classes, set to 'info' by default. Each class has a separate logger instance which inherits the logging level of its parent.

async with Airtable(logging_level="info") as at:
    base = await at_user.get_base("<Base ID>", key="id")
    table = await base.get_table("<Table Name>", key="name")

• Logging level can be changed for each class instance:

table.logger.level = "error"