Activity dots can now be shown on each cell

This is the long-awaited followup to #2 - really sorry about the delay. I ended up putting this project on hold, and I'm only just getting back to it.

Anyway, this pull request lets you specify whether there should be an activity dot on a given date. I've taken your suggestion on implementing it, and it appears to do the trick nicely.

Screenshot:
![screen shot 2014-02-26 at 17 44 29](https://f.cloud.github.com/assets/34273/2273247/af768220-9f0d-11e3-9989-22f4e040f144.png)