Add GBFS attributes to station information

[edwinvandenbelt]

We should add these properties to /operator/station results:

- num_bikes_available	Yes	Non-negative integer	Number of bikes available for rental. Number of functional bikes physically at the station. To know if the bikes are available for rental, see is_renting.
- num_bikes_disabled	Optional	Non-negative integer	Number of disabled bikes at the station. Vendors who do not want to publicize the number of disabled bikes or docks in their system can opt to omit station capacity (in station_information), num_bikes_disabled and num_docks_disabled (as of v2.0). If station capacity is published, then broken docks/bikes can be inferred (though not specifically whether the decreased capacity is a broken bike or dock).
- num_docks_available	Conditionally required(as of v2.0)	Non-negative integer	Required except for stations that have unlimited docking capacity (e.g. virtual stations) (as of v2.0). Number of functional docks physically at the station. To know if the docks are accepting bike returns, see is_returning.
- num_docks_disabled	Optional	Non-negative integer	Number of empty but disabled dock points at the station.
- is_installed	Yes	Boolean	Is the station currently on the street?true - Station is installed on the street.false - Station is not installed on the street.
- is_renting	Yes	Boolean	Is the station currently renting bikes?true - Station is renting bikes. Even if the station is empty, if it is set to allow rentals this value should be 1.false - Station is not renting bikes.
- is_returning	Yes	Boolean	Is the station accepting bike returns?true - Station is accepting bike returns. If a station is full but would allow a return if it was not full, then this value should be 1.false - Station is not accepting bike returns.
- last_reported	Yes	Timestamp	The last time this station reported its status.

[edwinvandenbelt]

instead of num-bikes-... it's now num-assets-...