Better auto-detect

[natevw]

Right now the only "autodetect" is to fallback to "http://localhost:5984" as the server when no URL is provided. However, this could be extended to cover other common [browser-side] use cases:

• if location.path includes "_design/" it's a pretty solid bet what the server/database is [which should be picked!?]
• otherwise we could be an attachment in a doc, or a _rewrite and virtual host [what to do?]
• otherwise (and if not a browser context at all), only then would the default port on localhost make sense

[natevw]

Brainstorming: perhaps user can provide, instead of a url string, an options object e.g. {context:"vhost", target:"server"} as hints for the autodetect as to the current situation and the desired database vs. server target.