elm logger

moved this from the Logging repo (aspnet/Logging#38), removed the Mvc dependency from the sample, and fixed scoping
I was considering doing that instead but wasn't sure of the efficiency cost. I also use `AllMessages` when calculating the total log count which happens every time an activity is added (once per request) and without it I would have to walk every activity, which actually won't end up being too big because of the capacity limit.