Add login

This changeset adds user-authentication awareness by way of the Devise gem.

See https://www.pivotaltracker.com/n/projects/1172184/stories/80648080

See https://github.com/dpla/heidrun/pull/3.  This mounts KriKri inside of Heiðrún and shows off its simple signup, login, and account-management pages.

I'm assuming that certain things, like token-based authentication and fleshing out the ACLs, are best implemented in other stories.  Right now, the basic request for logins is being satisfied.  The application controller's before_filter requires all KriKri requests to be authenticated.

The AclController is deliberately left incomplete, since another story will be created to deal with access control, probably after we've thought about our needs some more.

Interesting.  Avoiding upward dependencies seems like a good thing.

My main question is this:  say an outside developer installs our engine in their application.  Then we release a new version that includes a new migration.  The next time the outside developer runs `rake db:migrate` on their application, will our new migrations be applied to their database?  And is that a good thing, or might it mess them up?  I may need someone with a better understanding of how engines work to weigh in here.

Also, for what it's worth, the devise and blacklight migrations are copied forward into the host application's `db/migrate/` directory.  We don't have to do what blacklight and devise do, but it's a small plus to have consistency.