Added two functions, readMemory and writeMemory, for accessing the
battery backed RAM on the DS1307.
Fixing issue where the current implementation of RTClib.cpp prevents the use
of a global variable i in a sketch.  Instead of having a global int i, the
calls to send/write can be called with an explicit cast of 0 to a uint8_t.
Added functionality to access the SQW/OUT pin on the DS1307
Merge branch 'master' into feature_ds1307_sqwpin
Switched from 1 second to 5 seconds to give more time to see frequency