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.
Rename example .pde to .ino
Merge pull request #24 from mic159/master

Using PROGMEM to reduce RAM usage
Merge branch 'master' of https://github.com/adafruit/RTClib
Add README.txt
Add comment to example to show how to explicitly set the time.
Add square wave output support from dmalec in pull #11.
Merge pull request #26 from adafruit/square_wave

Add square wave output support from dmalec in pull #11.
Update keywords.
Add NVRAM functions from mabarnett0's pull #20.  Also add example of using NVRAM functions.
Merge pull request #27 from adafruit/nvram

Add NVRAM functions from mabarnett0's pull #20.
Refactor to handle old and new Arduino versions without code duplication, as suggested by #14.
Merge pull request #28 from adafruit/refactor_arduino

Refactor to handle old and new Arduino versions without code duplication as suggested by #14.
Add TimeSpan class and simple DateTime duration arithmetic support.
Merge pull request #29 from adafruit/timespans

Add TimeSpan class and simple DateTime duration arithmetic support.
Add secondstime

added missing secondstime function
Merge branch 'master' of https://github.com/adafruit/RTClib

# By Tony DiCola (8) and others
# Via Tony DiCola (6) and driverblock (1)
* 'master' of https://github.com/adafruit/RTClib:
  Add secondstime
  Add TimeSpan class and simple DateTime duration arithmetic support.
  Refactor to handle old and new Arduino versions without code duplication, as suggested by #14.
  Add NVRAM functions from mabarnett0's pull #20.  Also add example of using NVRAM functions.
  Update keywords.
  Add square wave output support from dmalec in pull #11.
  Add comment to example to show how to explicitly set the time.
  Add README.txt
  Rename example .pde to .ino
  Use __DATE__ __TIME__ from PROGMEM

Conflicts:
	RTClib.cpp
	RTClib.h