set up backend part of authentication with passport

Added passport authentication system on the backend, with connection to the database to check/create users.  I think this will work once we figure out how to pass the username/pw info from the front end.  

We should have a logout function, but I didn't get to that yet.  I think it's pretty simple, basically, from passport website: 

app.get('/logout', function(req, res){
  req.logout();
  res.redirect('/');
});

If you guys don't want to use passport, that's fine, I kept in in a separate branch so we can just drop it.  

It seems like it might be better to use token based authentication in a mobile app, in which case I think we can integrate json web tokens with passport, or just switch to a token based system.