319 logging actions

Implements #319.
This should be more specific; eg, `if cannot? :manage, :logs`. The idea of cancan is to move away from roles and toward permissions (if that makes any sense). Then roles and permissions are tied together in `models/ability.rb`

You can actually just write `authorize_resource, :class => false` at the beginning of the controller instead of creating your own `before_filter` and CanCan will automatically check for permissions involving `Log` whenever any of the actions in `log_controller` are called. Since admins already `can :manage, :all` you don't  need to bother changing anything in `ability.rb`