Add "num_usable_bikes" and "num_usable_spaces" to station_status.json

[albloptor]

This change tries to fix the limitation the current spec has, where it's not possible to know how many bikes are there at a station when is_renting = 0.

In my opinion, as exposed in issue #94, is_renting should be independent to num_bikes_available. In the same way, is_returning should be independent to num_docks_available.

Because changing the spec would break existing clients implementing it, I propose adding num_usable_bikes and num_usable_spaces to the spec.

These two new fields, in conjunction with is_renting and is_returning, arguably make num_bikes_available and num_docks_available redundant, but keeping them guarantees backward compatibility.

What we achieve with this spec change is to know how many bikes and spaces there are in a station regardless of the is_renting and is_returning flag.

This allows clients to:

• Keep all the current GBFS functionality, but providing the extra features below.
• Plan their journeys before the beginning of a station's renting hours. If a station starts renting at 7am, they will be able to see how many usable bikes there are at the station before that time. The current spec would display 0 bikes until 7am.
• Visualize number of bikes in each station on a map, regardless of the renting/returning hours.

[jcn]

Closing this as there appears to be consensus in #94 that adding additional fields does not actually solve the underlying problem, and that the better course of action is to fix/clarify the existing fields.