Handle URL Encoded Form Data

[toddgeist]

@rwu2359 points out we don't handle form data very well in #11

We need to add another section for FormData, just like Params. We can't use params, since that maybe used as well in the same request. We probably can't do Multi-part here.

• add new form-data table
• change Data tab to accommodate a new UI for setting form data
• add new section to Save and Export
• add new section to Restore
• make changes to HTTP Script