Adds basic authentication support

Basic authentication support needs a working /_config too for admins, so that's included. Not yet included is support for /_security and the access checks that come with that. I thought the PR was large enough as-is.

Support for /_security would be my next step, it should be pretty easy since all the logic is already implemented in the [pouchdb-security](http://npmjs.org/pouchdb-security) plug-in. This includes a Fauxton update which enables login/logout support. The log out link overlaps with the logo, unfortunately. I didn't try to solve that one since I don't know that code base at all. /_security support isn't there yet as I said before, but validation functions and show/list/update can see the userCtx object.