add interrupts, EEPROM and bluetooth logic

- allow the device to go to sleep, it will be woken up either
  by a serial event or by a pressure on the sensor
- use the eeprom to store/load id and data
- add the bluetooth side, first version:
    * commands are in the form "$" + char, with one char per
      possible command (echo E, get hugs H, etc)
    * commands with arguments are: $I@arguments
    * answers are preceded by "@"

TODO:
 - use a better protocol: differentiate between the answers by
   adding the initial command (@ + char + answer ?)

Note:
 - for the interrupt on the pressure sensor to work, the sensor
   should be on pin 6 and the reference voltage on pin 7
 - the default reference voltage is 1.1v, which is too high
   => use resistances to have a ref of ~0.2 v
 - 0.2 v: resistance on the + side is ~24x higher than the one on the
   ground side (and the input is taken "between" them, of course)
   formula : v_out = v_in * R2 / (R1 + R2)