Refactors coin_slot.ino. Windows instructions:

Setting up the Arduino IDE for the Trinket in Windows was kind of
hellish.

- Download the Arduino IDE v1.05 & install.
- Follow Adafruit's "slow way" instructions for setting up the
  Trinket with the Arduino IDE.
  (http://learn.adafruit.com/introducing-trinket/setting-up-with-arduino-ide#the-slow-way)
- Download Adafruit's Windows driver for the USBtinyISP
  (http://learn.adafruit.com/usbtinyisp/download).
- In Device Manager, right-click the Trinket, update driver, and
  point to the place where you unzipped the Windows driver.
- Grab the Bounce2 and TrinketKeyboard libraries and put them in
  folders in C:\Users\<user>\Documents\Arduino\libraries.
  (https://github.com/thomasfredericks/Bounce-Arduino-Wiring
   https://github.com/adafruit/Adafruit-Trinket-USB)
- Start the Arduino IDE.
- Make sure Tools => Board is set to "Adafruit Trinket 16MHz".
- Make sure Tools => Programmer is set to "USBtinyISP".
- Verify (compile) the sketch.
- Press the Trinket's reset button.
- While the Trinket is flashing red, hit the Arduino IDE upload button.

Hopefully that all works.