Key reading factored into separate function.
Measure and send time to press 2 chosen keys in order.

* One byte command selects 2 keys. First key in lower nibble and second
in upper.
* Time is returned as milliseconds in the form of 4 bytes (a long).
Read time between keypresses over serial.
Removing boolean hacks fixes missing top 2 keys.
Present graphic prompt and save data to csv file.
Ignore Processing and Arduino sketch property files.
Ignore data files.
Fix intermittent failure of serial sync.
Graph data files on key sequence timing.