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.
All `<input>` elements have a `value` property, so we can check the `tagName` to determine if we examine `value` or something else.  The other elements to handle are:
 * `<textarea>`: (`value` property)
* `<select>`: `value` of the child `<option>` with a truthy `selected` property

We also need to ignore `<input>` elements with a `type` of "reset", "button", and "image" (in addition to "submit"). 

Finally, all fields with a `disabled` attribute should be ignored by the parser.