This is only temporary, until such time that we release Mapbox GL
on CocoaPods directly, when they will host the Podspec. For now,
it needs to be in the repository root to be usable as a non-main
CocoaPods source.
This podspec was originally lifted from mapbox/mapbox-ios-sdk, which was structured and built quite differently. We could have the podspec build this repo as is, using scripts/package_ios.sh, but that would require developers to install the same version of Xcode as Travis is using (see #977). So this podspec instead pulls a prebuilt library. Much simpler.
Also package a LICENSE with the archive to satisfy a CocoaPods warning.
Fixes#829.
