Handle time stamps on precip data.

[dblodgett-usgs]

We either need to wipe and rebuild precip regularly or implement time stamp checking. This is very rudimentary. #12 (diff)

The page that we are checking does have time stamps for files: http://www.srh.noaa.gov/data/ridge2/Precip/qpehourlyshape/2017/201709/20170919/

So maybe we could actually check them, but that would be more fun html scraping.

[aappling-usgs]

fetchTimestamp.url already does some timestamp checking (though i'm a bit worried about the timezone that's written...there might be a vizlab fix in our future). so you could use that rather than doing your own html scraping.