Add default values for distance and max.

[riju]

Fixes #3, #4.

[anssiko]

LGTM, thanks!

I think we want to leave #4 open for discussion to gather further feedback on whether we should make the distinction between near boolean and distance in centimeters more explicit. If they're exposed via the same ProximitySensorReading object, they cannot be unbundled in terms of permissioning etc. Sensor access is requested per sensor instance (in this case ProximitySensor), see https://w3c.github.io/sensors/#request-sensor-access