always update check status on event

This required some changes to the calculation of outages in the API's EntityCheckPresenter, so that similar states would be consolidated in the output from that method. I've reviewed the rest of Flapjack's code and the data structure docs and think there should be no other side effects from the changes.

There's also some minor refactoring in executive.rb, attempting to simplify the code a bit.

Fixes #221.