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
@mintsoft wondering if we should be a little more clever and make sure there is actually content that needs to be decoded? For example "url decode tool" or "url decode online" would still return here which is unnecessary.

Or at the very least compare the remainder with the decoded string and if they're the same, don't both returning the result?