Allow librarians to edit all items.

Conflicts:
	app/views/items/show.html.erb
	db/schema.rb
Allow librarians to make other members a librarian.

Conflicts:
	app/assets/stylesheets/main.css
	app/helpers/application_helper.rb
Allow librarians to become normal members again.
Make sure members cannot make themselves a librarian.
Add tests for librarian.
Refactor librarian feature.
Expand README and explain librarian feature.
Allow to configure the librarian feature.
Merge pull request #24 from wvengen/feature/librarian

Allow librarians to edit all books.