Support for displaying data attribution

[bcamper]

We need to determine the most appropriate way for Tangram to support displaying attribution for data sources (OpenStreetMap but also others).

Some options:

• Do nothing, leave it as a client responsibility. I like the idea of adding a per-data-source attribution field to scene files regardless, so they they are a self-contained document of what's needed for their proper usage. But Tangram could simply provide these attribution strings (individually or concatenated for all sources) to the client.
• Implement attribution display at the platform SDK level, which avoids complicating the Tangram C++ core (think of wanting control over for your attribution font, etc.). The platform SDKs will be the primary way the library is publicly supported/distributed, and if you're using the underlying C++ API, you're probably doing something custom (such as Giflantro), and doing your own attribution display feels reasonable to me.
• Implement attribution display in the Tangram C++ core. I'd prefer to avoid this as it's a bit of a distraction and requires additional text rendering logic out of band from the label rendering process. It's doable though, just not something we want to take on now.

See mapzen/eraser-map#117 for background.