Add frequency option

closes #3. seems to be feature-complete, but could definitely use some refactoring.
The methods in this module are starting to look repetitive... the clarity of the methods make sense, but it seems like they could be refactored in order to make far fewer of them. I.e. rather than saying `daily`/`monthly`/etc., just pass around a `duration` so we don't need three of the `*_values` and `*_average` methods. We can certainly revisit later.