SpecDummy: add Language model.

This model will be used to test with extra options.
Activator: dynamically build 'active' class method.
Activator: add more dynamic functions.

Dynamic functions so we can properly use our set `Activator` field for
everything.
Activator: make code more strict.
Readme: update with `activator_field`

Give some additioi=nal information about the `activator_field` option.
Merge pull request #5 from jelmersnoeck/jelmer/activator-field

ActivatorField: make it possible to specify field.