Fix #57
Fix #57

This refactors the GPIOOutputDevice to have an internal `_write` method
similar to the `_read` method used to grab the value of a GPIO device.
This is used for simple 1s and 0s until we get to PWMOutputDevice which
replaces the value with the PWM device's duty cycle level. As a result,
all the DigitalOutputDevice base methods (including blink) "just work".

This commit also, for the moment, removes min_pwm and max_pwm because
I'm not 100% certain how they should interact with the notion of
is_active at the moment. They'll probably get added back in at some
point but I need a bit more time to think about it!

RPi-Distro/python-gpiozero,lurch/python-gpiozero,waveform80/gpio-zero,MrHarcombe/python-gpiozero