Use a single database for all Ruote objects

Hey John,

I've implemented a single database storage on CouchDB, juggling your code around and making `ruote/test/functional/storage.rb` pass. No big optimizations yet, except of course for the slightly less memory usage and one less `Thread`. I'll work on optimizing this storage in the future.

Anyhow, what do you think? Could be this ready to be merged, or do you require also a migration procedure from the former database structure?

Thanks!

~Marcello