Per-service static_fields. Resolve #659.

[ryneeverett]

Add a static_fields option to Common Service Configuration Options. This
enables users to differ behavior of core UDA names (e.g., "description")
amongst services.

The tests fail because we're currently able to test db.synchronize using regular dicts as issues whereas in a real bugwarrior pull these are Issue instances. This PR is awaiting a much-needed refactor of db.synchronize which will break this monstrous function into more testable components.

[ryneeverett]

While restructuring db.synchronize would still be a nice step, I think a simpler architectural improvement which would also unblock this for testing purposes would be to make the Issue class private to the services subpackage which can return simple dicts instead. This not only makes testing easier but eliminates unnecessary coupling between services and db.