Add events link to navigation ref #42
Add attachment model
Merge branch 'Biber/feature/navigation' into dev
Fix attachment fixture, make title and url required
add assignee_id to tasks
Add attachments on task edit/new pages
Fix attachments#index test
add done-field to db scheme; add checkboxes for read, update and create; start to build some kind of panel to show tasks as shown in the mockup
Add gems devise_openid_authenticatable and rack-openid to project, ref #9
Change devise user model and login, ref #9

  * add string identity_url to model
  * remove undesired devise options (like registration)
  * change sing_in view to openid only
Add logout button to navigation bar if logged in, ref #9
Restrict view access to signed in users only, ref #9

  * redirects to sign_in page if not logged in
Redirect user to their previous called page after login, ref #9
Restrict OpenID provider to HPI, ref #9
Add a sign-in-call to all controller tests, to fix all crashes, ref #9
fix association and add dropdown
fix saving user_id
fix layout
add blank instead of prompt
Add group class ref #20