Release 0.6.0

**Important changes**:

- The way users configure logging has changed. No longer are most users
  expected to be familiar with how to configure python's standard logging
library with a config file. Instead we've abstracted out the setting of log
level, format, and destinations to make these settings more accessible to
users. Expert users familiar with [the logging config file format][logconffmt]
can still make tweaks.

Summary of changes:

- Make logging configuration easier for the user.
- Add UML diagrams to documentation. They can be found in docs/source/images/
  and regenerated with `make umlsvg` in docs/.

[logconffmt]: https://docs.python.org/3/library/logging.config.html#logging-config-fileformat