feat($urlencoded): Add support for urlencoded and GET requests

The goal is to solve #8 and then #9. Right now this just adds support for urlencoded GET requests. There are probably a lot of holes in this implementation (cross-browser testing still needs to happen...), but I will aggregate my commits here. Feedback welcome.
Interesting idea, but may not be worth the effort & complexity.  It's likely more intuitive/common to simply modify the field values via the DOM API.