add action pipeline support

This allows `action` to be an array of middleware instead of a single handler. For example, we have middleware to do oauth2 scope checks, role checks etc.

```
var operation = {
  spec: {...},
  action: [
    authorize({ scope: 'rwx:verification', audience: 'api' }),
    function (req, res) {
      // do work here   
    }
  ]
};
```

@mgutz even though this is just formatting, I would prefer to see it in a separate commit.