Add support for distance sensors

[blaues0cke]

It would be great to detect whether a user is in bed. To make this possible we need to attach some sensors to the bed, like distance or weight sensors. This also relates to #4.

[blaues0cke]

@ottojo You may have some ideas for that?

[ottojo]

My idea was to mount an upward facing ultrasonic/infrared distance sensor under the bed to detect whether the user is in bed. Another option would be to put the bed on force sensors, but while more reliable this would also be more expensive.

[blaues0cke]

Okay, maybe a photocell would also be possible? Sensors on the ground are a problem for my roomba 😡