Add Support for a Subtitle. Fixes #9

Hope this solution is good/ok.

The subtitle label has custom options, so that the subtitle can appear different from the notification text.
Both labels are vertically centered together.

![screen shot 2014-02-18 at 22 07 05](https://f.cloud.github.com/assets/298062/2201256/97ac65e6-98ec-11e3-9102-bb7980a09ff3.png)