Reuse existing short codes

Since Shorty is a public URL shortening service all short codes are
effectively shared. If two users try to shorten 'http://google.com' they
should both result in the same short code.

Currently there doesn't appear to be a good solution for doing a high
level of URL normalization so this is pretty sensitive. Case, characters
of non-semantic importance, and probably some other things could cause
effectively identical URLs to end up with different codes. I don't think
this is a big deal, the main purpose is just to combine these short
codes when possible and easy.