Development of issues #1 and #2

* Changed `connection` to `connection_link` as discussed in #1 
* Changed open public transport vocabulary to open transport vocabulary as discussed in #2.
* Broadened the scope of a `stop_point` and added properties
* Added semantics for an `agency` and a `vehicle`
* Extracted all GTFS notes from the navitia.io start doc and put them in mapping_to_gtfs.md

If we get a :+1: from 2 different people, I will press "merge pull request".