Make the timestamp field configurable

Make it possible to use a field, e.g. @timestamp, for metrics sent to
Graphite instead of current timestamp. Useful when backfilling or
getting nice and complete graphs.

Thanks to @mfournier for encouraging me to submit this PR.
I'd prefer `@timestamp` being the default too, but I'm hesitant to
change the default because of principle of least surprise when people
upgrade to 1.5.