This change makes ".xkcd" module more robust:

- don't allow negative numbers which accesses a comic number lower than 0
- don't allow the 0th comic when randomly checked
- allow also a "+" and "#" as a sign
- don't show an error when accessing #404
- don't show an error when no search.google_search() returned None
Merge pull request #587 from xZise/patch-1

More robust number check