Add alarm based button debouncing

[ar0n]

Looks like the quality of the buttons I used is not that great, so I've added new debouncing.

Resolves #14

[alephbias]

This does fix the problem, however there's a slight bug in it.

With the correct timing, it's possible to double click the ARM button and get into a state where the Arming LED is off, but the HV LED is on and the device can successfully glitch. To get out of this state involves clicking the Arming button again, waiting for the Arming LED to turn on, then clicking it again to dis-arm the PicoEMP. The sequence to try is:

• Press the ARM button
• Wait a brief moment (potentially slightly longer than 80ms?)
• Press the ARM button again
  After a few tries of this, this weird state will be entered. If you can't get it to happen, try modifying your delay before pressing the button again.