Sets a single PWM channel.

def set_pwm(self, channel, on, off):
        """Sets a single PWM channel."""
        self.i2c.write8(LED0_ON_L+4*channel, on & 0xFF)
        self.i2c.write8(LED0_ON_H+4*channel, on >> 8)
        self.i2c.write8(LED0_OFF_L+4*channel, off & 0xFF)
        self.i2c.write8(LED0_OFF_H+4*channel, off >> 8)