Authenticate current_user and select current_team

[mediafinger]

Implements authentication with email / password credentials

• Closes https://github.com/mediafinger/yournaling/issues/14
• Users can now login and logout
• per default a Guest User ("NullObject") is being used
• Users can select one of their teams, which will be stored in the session
• helpers for current_user, current_team and current_member available
• this will allow to scope (almost) all actions to a team
  • therefore the team can become the owner of all entities (like pictures, locations, etc...)
  • the user will only be stored as "uploader" or "creator" or "editor"

TODO

• [x] fix broken request specs / add spec helpers
• [x] add a few request specs
• [x] use "null object" guest user?
• [x] allow guest users on certain pages (or just stick to current_user = nil ?)
• [x] avoid "Logout Guest" link, instead show "Login" button