Add `i2c_scan` method to `DMFControlBoard`, ...

 - Add `i2c_scan` method to `DMFControlBoard` Python class.
 - Use [`eeprom_update_byte`][1] instead of [`EEPROM.write`][2] _(which
   uses [`eeprom_write_byte`][1])_ to avoid unnecessary write when byte
   has not changed.
  - __NB__ This likely slightly affects EEPROM write performance since a
    read needs to be performed, followed by a write.

[1]: http://www.nongnu.org/avr-libc/user-manual/group__avr__eeprom.html
[2]: http://arduino.cc/en/Reference/EEPROM */
Decouple feedback processing from DMFControlBoard

Add `FeedbackController` class, which decouples feed-back processing
from `DMFControlBoard`.