Reading values: Add functions to read sensors

[pfiaux]

Currently we expose 2 read functions:

• getRecipes(NEEOBrain)
• getRecipesPowerState(NEEOBrain)

As mentioned in #157 there's currently no easy way to read sensor values. Sensors should be readable implying:

• We can get devices from the Brain
• We can get sensors from Devices
• We can get sensor values from sensors

One way might be to write this as a "chainable" api: getDevice(NEEOBrain, deviceId).getSensorValue(sensorId), but there's lots of different ways this could be written.