Added the AStar32U4Prime library.
Added the AStar32U4PrimeLCD class and the LCDBasics example.
Improved the LCD documentation.
Added the AStar32U4PrimeBuzzer class and the BuzzerBasics example.
Added AStar32U4PrimtButtons.h and the Keyboard example.
Added the Buttons example.
AStar32U4PrimeButtons: Made the delay shorter because it only takes
about 0.5 us for the I/O pin voltage to settle.
Updated the Pushbutton library.

Also added an include for util/delay.h in AStarPrime32U4Buttons.h
because it uses it.
AStar32U4Prime: Added LED functions, power detection functions, and three examples for them.
AStar32U4Prime: Fixed a comment in USBDetect.ino to say bool instead of boolean.
AStar32U4Prime Buttons example: Use the green LED instead of yellow
since it will be easier to see if the LCD is plugged in.
AStar32U4Prime: Added README.md and added \brief descriptions for button classes.
fix typos in README.md and mention pushbuttons in features
SD card support:
- AStar32U4PrimeButtonC and AStar32U4PrimeLCD: Temporarily disable the
  SPI module using a new class SPIPause.
- Added SDTest example.

Also:
- Don't enable the pull-up for buttons B and C because it is not
  needed and can make things worse.
- Updated local_keywords.txt.
Merge remote-tracking branch 'origin/review/ryantm/readme'
be more specific about which green LED will be lit
remove extra printed space
AStar32U4Prime library: Made readBatteryMillivolts take an optional
pin parameter.
AStar32U4Prime: Added a big Demo program.
AStar32U4Prime: Updated components.