Removed Devise, added basic authentication

Here is what I did:

I created a migration to get rid of the fields that Devise left behind. I also got rid of the Devise files.
The basic authentication is there. Data sets, sources, and tags can not be modified or destroyed by non-users. It will just redirect you to the login page.

One caveat: If you login with the wrong password, it will give you a 'template missing' error instead of returning you to the login page. I cannot figure out how to display the error in the view using form_tag .

I know we talked about users only being able to delete their data sets, sources, and tags and not others', but I would rather set up Rspec and get some tests up before I tackle those issues.
I think it'd be nice to have a `login` method defined in `ApplicationController`, so that anywhere we want to log in a user, we just need to call `login @user`. Then if we decide we want to handle login differently (with a cookie, or storing more info in the session, or whatever), we only need to change one place.