Add support for webapi auth backends.
This adds support for custom auth backends for the webapi. API auth

backends are a simple class that pulls credentials from a request and

authenticates with them. They're designed to work for common methods

(like Basic HTTP Auth) and to be subclassable for something more complex

(such as one-time password tokens).
Multiple auth backends can be specified, and each will be tried until a

matching request is found. This means we'll be able to add Digest Auth

soon, in theory.

Tested authenticating with this. No problems.
Also tested with a custom, experimental auth backend for two-factor auth.

It worked as well.