Updated SdFat library to 20130313 for Arduino 1.0.4

This update fixes an issue where compiling software using the SdFat
library under Arduino 1.0.4 failed. The updated SdFat library includes a
new version of FreeRam() that has been modified to use Arduino's new
implementation of malloc(). See
http://arduino.cc/forum/index.php?topic=154033.0 for more information.