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.

In PR #18 (now merged to develop), I did almost this same thing in `install_generator.rb`.  I did:

`route 'mount Krikri::Engine, at: "/krikri"'`

Your way is better b/c it doesn't junk up the rout pathnames with `/krikri/`.  But when you merge to develop, someone will need to remove the method that I wrote from `install_generator.rb`