Indicate stability of terms

[pietercolpaert]

Using http://www.w3.org/2003/06/sw-vocab-status/ns#term_status you can indicate the status of a term defined. We should do this for every term.

Discussed with @rtroncy during ISWC2014

Proposition for linked GTFS:

• use the string "stable" only for predicates that have been implemented in https://github.com/OpenTransport/gtfs-csv2rdf's master branch
• use the string "testing" only for predicates that have been implemented in https://github.com/OpenTransport/gtfs-csv2rdf's development branch
• use the string "unstable" only for predicates that haven't been adopted by that have been implemented in https://github.com/OpenTransport/gtfs-csv2rdf yet