Automatically download and parse documents from USPS website via watir

[joeyparis]

Adding the ability to download and parse updated USPS User Guide documentation automatically could make this gem fully self-sufficient (assuming mild consistency with their documentation layout).

I've successfully got the files downloaded using Watir on the feature/watir-downloading branch; however, the format is surprisingly different from using the "right-click, Save as..." approach and much harder to parse reliably.