add basic Mapzen UX/SDK/API elements

[nvkelso]

Fixes #93 by adding:

• Data visualization global sort orders
• Default draw styles
• Basic user experience styles
• Transit user experience styles

Outstanding:

• Why isn't Tangram honoring RGBA alpha overlays anymore? Looks like this is broken in all house styles now /cc @bcamper
• The existing artwork assets are greyscale placeholders from Refill. @sensescape will colorize them in the shields sprint (binary AI file, what what). The sprites in this YAML should be renamed to match Bubble Wrap, but that should be done as part of her shields PR as it's part of the export process.

Does not fix:

• Language: https://github.com/tangrams/walkabout-style/issues/88
• Transit overlay: https://github.com/tangrams/walkabout-style/issues/89
• Buildings: https://github.com/tangrams/walkabout-style/issues/90

[nvkelso]

@bcamper This big blue polygon should be transparent, and was several Tangram versions ago:

To get to this state, you'll need to uncomment the mz_default_polygon source lines and zoom to San Francisco.

Looking at Bubble Wrap the same problem is happening, both for lines and polygons (see transit platforms).

[matteblair]

Nice, LGTM. Any reason to not merge this now?

[nvkelso]

Ready to merge now :)