Simplify API and examples

[danrubel]

• [ ] Provide top level gpio accessor for obtaining the initialized Gpio instance for interacting with the GPIO pins running on the Raspberry Pi.
• [x] Provide top level pin accessor for obtaining the initialized Gpio pin instances
• [ ] Provide a new Gpio(hardware) constructor for mocking
• [ ] Deprecate the Gpio.hardware = setter in 0.2.x and remove it in subsequent release
• [x] Remove mocking from the existing examples and provide one new example with mocking
• [ ] Move hardware API to dart:pi that is delivered as part of an sdk extension - see Simplify installation with prebuilt SDK with library

[danrubel]

For example, it should be easy to consume te the rpi_gpio package in order to drive hardware buttons and strips of LEDs.

import 'dart:math';
import 'package:rpi_gpio/rpi_gpio.dart';

Random random = new Random();
double next() => random.nextDouble();

void change(Strip led) {
  led
    ..red = next()
    ..blue = next()
    ..green = next();
}

main() {
  var led = new Strip(pin(1, output), pin(2, output), pin(3, output));
  change(led);
  new Button(pin(4, input), () => change(led));
}

[danrubel]

New API published in version 0.5.0