Support converting created dates to a local timezone using moment

Looks like it's much faster to use something like `moment(item.properties.created).tz(this.timezone).format()`: http://jsperf.com/moment-timezone-vs-timezone

(I was rooting for bigeasy/timezone, since it pretty cleanly uses POSIX time as the raw representation functional operations on top of that, but it's a pretty big performance gap).

We can define a timestamp formatter in the RowStream constructor, since `this.timezone` will exist for every item or none of them. Or at the very least, using the above form avoid creating an unneeded ISO8601-formatted string when we don't need it (which ought to be the norm).