Add support for ATTiny*4

ATTiny24 / 44 / 84 use TIM1_COMPA_vect instead of TIMER1_COMPA_vect for
ISR.
manipulate TCCR1A & TCCR1B directly

this way the code is 36 bytes smaller, and runs faster.

however the performance gain is trivial because we usally call begin()
only once...
arduinolize sei() and cli()

replace sei() and cli() with interrupts() / noInterrupts().
add a delay between outputs

each channels are delayed by 1 PWM clock cycle to prevent large
in-rush currents.
This make the life of the power supply easier.