[Feature Request] Add period at the end of error message

From UX perspective, a full sentence is to end with a period (see http://ux.stackexchange.com/questions/18671/), but this is not the case with Rails. In https://github.com/rails/rails/blob/master/activemodel/lib/active_model/locale/en.yml, we see that error messages are left hanging.

If the community agrees that we should add period at the end of error message in ActiveModel (and everywhere else where applicable), please +1, then I shall make a PR.

I often join with a comma, so having a period at the end would break that. But having it be an option wouldn’t be a bad thing.