Add support for retrieving Volt-Low bit

* Access to this bit is critical to know whether
  or not the RTC's Vcc dropped below threshold
  and possibly corrupted date/time.  Setting
  the time will automatically clear this bit.

Signed-off-by: Chris Evich <chris-arduino@anonomail.me>
Add clearVoltLow() member function

It doesn't matter if you set the VL_Seconds bit low or
high, it is always cleared when you write to it.  Added
a member function that only clears the bit, retaining
current date/time values.  Made setTime() to mask off
the VL_Seconds bit, so it's always cleared no matter what.

Signed-off-by: Chris Evich <chris-arduino@anonomail.me>
Merge pull request #7 from Yewess/volt_low

Support reading & clearing Volt low bit