Support optional dropdowns

This gives a "blank" answer for dropdowns that are optional.

![screen shot 2014-10-14 at 6 11 10 pm](https://cloud.githubusercontent.com/assets/4962198/4639186/5215dfbc-5408-11e4-8c07-054095e985b1.png)

Hmm, I guess you can also start from -1 if the response is optional so that the first valid response always has value zero. You can then check for empty response below.