UrbanDictionary fixed, required me to change http.py - Also minor nitpick with stock.py

Added referer field to http.py, as to make it possible to use a referer. I
don't know if this was possible before, and if it was it should be removed
and the appropriate changes should be made without the http.py change.

UrbanDictionary plugin was changed to use a referer because it didn't work
without it.

Stock plugin output was changed so the percent change has an actual
percentage sign after it.