refractoring to reduce power usage

in scales.DSN:
* off button active in HI
* move buttons to higher nibble of PORTB
* add some NMOS transistors connected to the buttons to trigger an interrupt regardless of which button is pushed
* interrupt enabled on lo-to-hi transition (I think)

in scales.c:
* move button handling from main loop to interrupt handler
* sleep mode PWR_DOWN in INPUT state because an interrupt awakens it
* refractored state changing when WEIGH button is pushed
* moved init code to reset()
* in reset(), do not reset stored number ; do not set state (that's set depending on how reset() was called)

for great glory of programming chips and fish!