Added ability to skip checking on certain paths

In our setup we are serving our api and site on different domains but using the same node application.  

http://site.com
http://api.site.com

So doing http:/api.site.com/api just is stupid looking.

With this fix you can now specify paths to skip.
`app.use('/', expressJwt({ secret: app.config.session.secret, skip: ['/token'] }));`