Added URI encoding/decoding for notifications description.

I found a bug earlier that without encoding the description of a new notification it will fail but still create the notification server side, I have submitted this bug to Withings already as these show up with callback_urls that are nil, but are still in the list. 
This pull request adds URI encoding and decoding to the description. The examples in the readme regarding notifications are not possible as they will fail with response code 432 "The signature (using Oauth) is invalid.", because the description is not getting URI encoded.