[Geofencing API] Beginning of a Specification

As Giri mentioned, I just uploaded an initial version of a geofencing
specification to the geolocation-api github repository at
https://w3c.github.io/geolocation-api/geofencing-spec.html

It still has long way to go (and I'm pretty new to the whole spec editing
thing), but any feedback is welcome. Other than naming, this pretty closely
matches the simplified api proposal
<https://w3c.github.io/geolocation-api/simplified_geofence.txt> we
discussed back at TPAC.

Marijn