Participation is now polymorphic.

Both a member and a user can belong to an instance of Participation,
which basically means that both members and users now can participate to
an presentation.
Merge branch 'master' of github.com:Samfundet/Bedpres into polymorphic-associations

Conflicts:
	app/models/user.rb
	db/schema.rb
Notification can now belong to a Member.
Added ordering to User/Member #notifications.
A users notifications is destroyed with the user.
Merge branch 'master' of github.com:Samfundet/Bedpres into polymorphic-associations

Conflicts:
	app/controllers/participations_controller.rb
	app/models/participation.rb
	app/models/presentation.rb
Only show notifications if actually logged in.
Bunch of variable renaming.
More consistent word convention.

The Then step said 'Then a user named ..', so I thought the Given step
also should say 'Given a user named ..'.
Step definitions for attendance supports members.
Adding participation tests for members.