/api/recentCrashes pagination

[tracernz]

@dustin, currently no way to get older crashes, can we do something like this (totally untested/written in github comment editor 😁)?

func handleRecentCrashes(w http.ResponseWriter, r *http.Request) {
	c := appengine.NewContext(r)
	start, err := strconv.Atoi(r.FormValue("start"))
	if err != nil || start < 0 {
		start = 0
	}
	q := datastore.NewQuery("CrashData").Order("-timestamp").Offset(start).Limit(50)
	res := []CrashData{}
	if err := fillKeyQuery(c, q, &res); err != nil {
		log.Errorf(c, "Error fetching crash results: %v", err)
		http.Error(w, err.Error(), 500)
		return
	}

	mustEncode(c, w, r, res)
}