redirect to original url after login

Added redirect to original page feature to hackathon-starter. I used the query string with param `r` to pass original url. 

`postLogin` controller handles them and redirects to after successful login, On unsuccessful login it again passes them to login page. 

I also added flash message  "You're not logged in" to show after redirection. 

To add current path directly to links I exposed `req.path` variable to locals so we can use that to modify Login links in `navigation.jade`.

- Express decodes URI params by default