Facility

Rather than having a hard-coded facility of `GELF` (which, from my understanding of the spec can be left blank and should be set to `GELF` *by the server*, not the client) I suggest we make it configurable. So I added `Facility` configuration property to the appender config. That way it's easier to distinguish multiple applications and to group logs together (ProcessName will be w3wp for all websites so less than ideal).