Support parsing a protocol string in shards.yml

This patch allows octopus to support full database urls in `shards.yml` instead of having to spell out all the parts, like you can in a rails `database.yml`. This will be especially useful for us on heroku, where I already have the database url handy and I can just ERB it in there.

I didn't add support for strings in shard groups as we haven't had a need for it yet. The three examples that are failing were failing before this patch, not sure what's up there.

Thanks for the great work!