use millis() and simpler time logging

NTP server is only queried once, in the setup() function.  Also, time is
recorded on the SD card as seconds from Jan 1 1970 UTC.  So, no
formatting and no time zone.  This takes the formatting load off the
Arduino and bugs the NTP server less.

I'm planning on adding a Raspberry Pi web server that will handle
collection, formatting, validation, and Wunderground uploading of the
weather station data.  I'm also planning to add a RTC module to an
Adafruit CC3000 WiFi shield for more reliable time keeping as well as
wireless capabilities.