Added commands for averaging

My very first pull request.  Go easy on me.  ;-)

This change adds four new command calls that allow switching voltage and current measurements between single-sample and 128-sample averaging.

+  void setAmpInstant(void);  // returns to single sampling
+  void setAmpAverage(void);  // changes config reg to 128-sample averaging
+  void setVoltInstant(void);  // returns to single sampling
+  void setVoltAverage(void);  // changes config reg to 128-sample averaging

I've tested these with my local project and found them to work correctly.

I also couldn't resist tweaking the README just a bit to make it more descriptive.

Thanks,
Chip