URLDecode: Adding new goodie to replace closed source version

Basically a copy of the existing URLEncode goodie with a few tweaks, should close this off: https://github.com/duckduckgo/zeroclickinfo-goodies/issues/478 as Rot13 already exists.

Includes a test explicitly for XSS. 

![image](https://cloud.githubusercontent.com/assets/978048/4179485/6a8c06d2-36c8-11e4-9b63-fe21d7c30ecf.png)

 cc @jagtalon @moollaza
@jagtalon that was the intention!