Add Time Library

From http://www.pjrc.com/teensy/td_libs_Time.html
Add TimeAlarms library

From http://www.pjrc.com/teensy/td_libs_TimeAlarms.html
Fix compile errors under gcc-avr 4.7.2
Add DS1307 library

From http://www.pjrc.com/teensy/td_libs_DS1307RTC.html
Add DS3231 library