Improved login and welcome nav

Josh, I'm not quite good enough with rspec to really do TDD in Rails yet, but this seems to roughly work. There's one bug that I know about that I couldn't figure out. If you log in, then View a Meetup, the Meetup name in the menu bar links to a bogus URL (e.g., `http://localhost:3000/meetups/5/5`.)

Will work on other things (the Meetup listing UI, probably) when I next have time to work on this.