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)

I ended up giving it the number that is one more than the last valid response. That way the first valid response always has value zero, and my numerical regex that I'll need to write later to analyze the data won't need to handle a negative number.