Adds auth package with token auth backend

Adds an interface for AccessController which handles verifying that a request has sufficient access for the current operation.

Implements a "token" AccessController which handles bearer tokens in the form of signed JSON Web Tokens.
```
// ServeHTTP prepares the request to conduct the appropriate challenge
// response. For most implementations, simply calling ServeHTTP should be
// sufficient. Because no body is written, users may write a custom body after
// calling ServeHTTP, but any headers must be written before the call and may
// be overwritten.
```