Display bus stops

[Teester]

Fixes #28

This pull request displays bus stops at z18 and their labels at z19. I've set visible: true for both, but you may want to use a global parameter to allow easy switching on and off.

[nvkelso]

Closing in favor of #31.

[nvkelso]

The key bug in the stylesheet was needing to include text: { optional: true } to improve collisions (to not require the text display – if the text couldn't display before then the icon would also not show.

I also bumped up the icon size like you suggested, but gradually – and renamed the sublayer later to earlier. That way the visibility all flows instead of needing override the global a couple times.

[nvkelso]

@Teester Thanks for your PR and issue, we appreciate your help to improve the Cinnabar style :)