Deprecate credentials in url

[YakovL]

Overview

This is a follow up for https://github.com/apache/couchdb-nano/issues/174 Docs should stop suggesting the http://username:password@127.0.0.1:5984 syntax for connection and suggest using .auth instead.

Testing recommendations

I'm not sure yet if we only have to update readme.md, but at least going through it with a simple search for pass/@ is helpful to find other places to be modified.

Checklist

• [x] check whether there's any related PRs;
  • [x] ask at #280 to remove the usage of such url
• [x] check if docs are stored in other places than readme.md;
  • no, examples/ don't contain any auth either; haven't reviewed all the autotests though
• [x] find all the places to be rewritten;
• [ ] update content:
  • [ ] below the header "TypeScript"
  • [ ] below the header "nano.db.replicate(source, target, [opts], [callback])"
  • [ ] below the header "nano.db.replication.enable(source, target, [opts], [callback])"
  • [ ] below the header "nano.db.replication.query(id, [opts], [callback])"
  • [ ] below the header "nano.db.replication.disable(id, [opts], [callback])"