Add readInterval param to configure Tinker read.

Allows end-users to configure the interval used for the Tinker firmware
digital/analog read API calls.

e.g.

Cylon.robot({
  connection: {
    name: 'spark',
    adaptor: 'spark',
    accessToken: 'adf',
    deviceId: 'adfa',
    readInterval: 500
  },

  device: { name: 'sensor', driver: 'analogSensor', pin: 'A7' },

  work: function(my) {
    var analogValue = 0;

    every((1).second(), function() {
      analogValue = my.sensor.analogRead();
      console.log(analogValue);
    });
  }
}).start();