Implemented colors from 'klasse' parameter. Not configurable yet,
except for the colors.
Preparing release 0.2
Back to development: 0.3
The following changes to functionality are in this commit:

    - Removed hardcoding of options.

    - The cache script now creates all layer objects, but only downloads
      values for layers of which the latest value is used by something
      (coloring).

    - Coloring is done using the configuration in the ColorFromLatestValue
      model.

    - Percentile bars are done using the configuration in the
      PercentileLayer model.

    - Layers that are used for augmenting other layers (color layers,
      percentile layers) can now be hidden.
Fix trivial error in test
Use rest_framework instead of djangorestframework in testsettings
Work on showing units, fixing a bug with hidden layers and the cache script.

- Add a 'visible_criteria' method to datasources, that by default just
  returns chooseable_criteria(). This should be used to hide choices
  that shouldn't be shown to the user. The previous method of hiding
  them from visible_criteria was inadequate, because then they would
  be hidden from internal use too (e.g., the caching script couldn't
  find them either).

- Add a 'unit' method to datasources, that optionally returns a string
  describing the unit of the data represented by
  them. Grondwaterinbrabant-site issue #10.
Remove unnecessary test.
Preparing release 0.3
Back to development: 0.4
Extra documentation, more tests
Add some tests, remove dead code.
Add tests, documentation
OptionList now ensures that its members occur only once. Uses a frozenset internally. Could be renamed to OptionSet.
CombinedDataSource now does the right thing when ChoicesMade is set.
Add tests, remove unused code
Add tests, documentation
Add tests, make API a bit nicer
Add legend functionality for colored locations
Preparing release 0.4