Improved for education and added power function

Hello,

This library was hugely helpful for me to get started using the INA219 board, but not without a few challenges understanding what it is doing.  So I updated it as I figured out some of those challenges, in hopes that it will be even easier for the next person to learn from.  :)

+ Added a bus power function.
+ Added all of the current and power ADC sampling settings from the data sheet.
o Updated existing functions for consistency and ease of learning from them.
o Resolved how the multiplier works for all functions and eliminated as much floating point math as possible by calculating in smaller units.
o Updated the example with the power function and saved as a .INO file.
o Lots of syntax and comment changes for for easier learning.