Allow ARNs to retrieve region and account from Boto3 session

A boto3 Session object has the `Session.region_name` field, and can call `Session.client('sts').get_caller_identity()` to discover the account in which it is operating (this call has no authorization check, so it is always allowed). When creating an ARN class, I should be able to tell it to use either the default session, or a session I provide, to discover these values (subject to whether the ARN needs those values, as covered in #7).