This commit fixes all warnings created by flake8 by removing unused
imports and variable assignments. Some wrongly indented lines are now
indented correctly.
This commit adds Meta information and __str__ methods to all DNS table
models. The new methods are now covered with new tests. The new
constants DNS_DOMAIN_METADATA_KINDS and DNS_TSIG_KEY_ALGORITHMS are
defined and used in the DNSDomainMetadata and DNSTSIGKey models. A
matching database schema migration is added.
Addresses #17
Jan Dittberner