add 12th Street Station, Oakland

this patch adds station code `0x0c` as [12th Street Oakland City Center][bart] station to the BART/Clipper Card table. The data should be good; I pulled the station ID from my phone and the lat/long from [OpenStreetMap][osm].

I did notice, however, that it seems reasonable to guess most of the missing BART station IDs: `0x0b` is probably West Oakland, the only station between Embarcadero (`0x0a`) and 12th Street, and so on. If you would accept such a patched based on inference rather than real-world data collection, I'd be happy to prepare one.

Thanks!

[bart]: http://bart.gov/stations/12th/index.aspx
[osm]: http://www.openstreetmap.org/browse/node/2150077203