Round down "now" to nearest 10 minute increment

This makes it cleaner to look at the times (since we use a 10-minute
interval) as well as giving a better chance of the selected row having
realtime data.

Also improves cache-ability which is a win.

https://www.pivotaltracker.com/story/show/83325086 [Delivers #83325086]
Merge pull request #49 from sealevelresearch/round-down-10-minutes

Round down "now" to nearest 10 minute increment