swagger: allow cross-origin requests

Add CORS middleware to the swagger app.

Add a configuration option allowing developers to disable CORS.

See also https://github.com/strongloop/strong-studio/issues/390

/to @raymondfeng please review
/cc @seanbrookes 
It should be:
```js
var corsOpts = typeof opts.cors === 'boolean'? { origin: opts.cors } : opts.cors || {origin: true, credentials: true};
swaggerApp.use(cors(corsOpts));
```