Change API related to time intervals

Introduce interval fn so we don't need multiple arities to support
defaulting to milliseconds while supporting other TimeUnits.

Motivation is that we have better reasons for multiple arities in
->x-subscriber fns.
Introduce ->keyed-batch-subscriber
Release 0.2.0
Merge pull request #2 from duelinmarkers/keyed-batch-subscriber-and-new-api

Keyed batch subscriber and new api