Get updated property details

This adds the `getUpdatedPropertyDetails()` method (http://www.zillow.com/howto/api/GetUpdatedPropertyDetails.htm). The only parameter this takes is the Zillow Property ID (zpid). So if this is an initial request for an address, you have to use something like the `getDeepSearchResults()` method first to obtain the zpid, then make the call to `getUpdatedPropertyDetails()`. If the zpid is stored in the database or obtained previously, it can be called directly. 

A couple unrelated issues:

1. I kept getting errors for `toQueryString()` that `this.id` was undefined, so I just passed it in as a parameter to the function. 

2. Also, for `getDemographics()` I was getting errors about too many parameters being specified. So in the documentation example, I just passed in zip code, which worked for me.