The iOS and OS X SDKs now share a non-UI test bundle. Moved the iOS UI test bundle into a uitest/ subdirectory and moved the OS X SDK tests into the darwin/ subdirectory.
Upgraded KIF to v3.4.2.
* 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.
An MGLOfflinePackProgressChangedNotification notification now includes the pack’s state and progress values, which match the values in the corresponding MGLOfflinePack properties. Implemented NSValue category methods to store MGLOfflinePackProgress inside a dictionary.
