Add library to read buttons

[margrevm]

Design:
Add Abstract class: ButtonAction <- SpecificButtonAction

• Contains an abstract "handle_button_press" function

Add function to register a Button action on a specific button

Add function that detects button presses every x ms and executes corresponding button actions.

Steps:

• Design
• Code
• unit tests
• Add documentation:
  • Hardware design
  • Software design

Use Bounce myButton = Bounce(2, 10); // pushbutton on pin 2

Example here:https://github.com/pfeerick/elapsedMillis/wiki