I've seen several issues where users followed platform-specific install docs, but were unaware of additional prerequisites spelled out in the top-level INSTALL.md. So let's try making each platform's installation documentation self contained.
* Main gyp files are now standardized as platform/<platform>/platform.gyp.
* Each platform gyp file defines appropriate loop_lib and headless_lib variables.
* Each platform gyp file includes mbgl.gypi, which defines base targets which may be useful to all platforms.
* CI targets are consistent across platforms: `make $(PLATFORM) && make test-$(PLATFORM)`.
* Renamed the "linux" test app to "glfw". It's now built in OS X CI.
* Android build flakiness is fixed.
* iOS CI builds the bench and iosapp targets.
* Mesa version is now in one place.
* CI scripts use bash "strict mode" and correct error handling.
* All build output goes to the build directory.
* Removed vestigial iOS/OS X/Android Travis scripts.
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/.
