Add ability to observe updates externally.

A light-weight notification interface has been added to allow external
observers to respond to updates to the chart. Additionally, the
relationship between a chart and its parent chart (if any) has been more
clearly defined.
Enable Sauce Labs / Karma testing in CI.
Merge pull request #11 from dabohealth/features/notify-observers-of-data-updates

Add ability to observe updates externally.