Use __DATE__ __TIME__ from PROGMEM

This modification will allow you to easily store the
__DATE__ and __TIME__ strings in PROGMEM to stop them
from wasting SRAM.
It uses the __FlashStringHelper, which is the same as
what Serial.print() uses and is provided by default in
Arduino.
Merge pull request #24 from mic159/master

Using PROGMEM to reduce RAM usage