This adds support for using cached styles that are stale. They're treated like changing styles; when the refreshed style changed compared to the one we've already had, we're swapping out the entire style, which might cause a slight flicker.
I regenerated assets.zip so that all file paths have an `assets/` prefix, as the Android AssetFileSource implementation asserts, and removed `TEST_DATA` from the paths.
Rewrote some copy to provide more context and link to more GL-related repos, including GL JS. Put the various SDKs in a table so we can show the various CI bot status images.
Integrated target list into the relevant documents. Rewrote large portions of iOS and OS X setup documentation to guide normal developers to more friendly places. Combined some Android documentation.
Moved SDK-specific documentation into platform/. Added a temporary readme in ios/ that points to the one in platform/ios/.
