url-hexify values in key-value pairs

Fixes an issue found in #129 and is based on that branch, so that must be merged first.

When the comment body contains a `+` (or indeed, any other meta-character like `&` or `;`), errors would arise when posting.
Thoroughly XD http://stackapps.com/questions/3950/sx-stack-exchange-for-emacs#comment11970_3950