User authentication

Add ability for users to log in and log out. Resolves #10 
@zporter Question somewhat related to this: I've seen that, in controller code, something like this is fairly standard:
```
if @user.save
  ...
else
  render :new
end
```
However, on a failed save, rather than go back to `/signup`, it goes to `/users`. While this definitely makes sense, I was wondering if there was a recommended method for displaying the correct errors while at the desired URL. I tried tinkering around with several things a day or two ago (i.e. messing around with the url parameters in my forms, trying a couple different things with routing (like for `users#create` or adding some `via: 'post'` stuff) and passing the current user state through a `redirect` rather than a `render`) but I ended up just tabling it to work on other stuff. Any of those that sound particularly good? Or is there a different solution that's better?