cmd/bosun: Add the d() func, i.e. d("1h")

d() returns the number of seconds for that opentsdb duration string. Currently this will just be nice when doing things like forecastlr(q("avg:6h-avg:os.disk.fs.percent_free{$filter}", "7d", ""), 0) / 60 / 60 / 24 or using the since function. However, we might consider changing the arguments to query functions to be scalars and use this function.
Merge pull request #599 from bosun-monitor/duration_func

cmd/bosun: Add the d() func, i.e. d("1h")