Support building from source via Carthage

[julianrex]

Currently this project does not support being compiled as a local repository by Carthage (as noted in the INSTALL.md).

When we can, we'll be able to specify the mapbox-events-ios framework as a dependency, i.e. it will help address the dependency issue noted here: https://github.com/mapbox/mapbox-gl-native-ios/releases/tag/ios-v5.6.0.

We should consider whether this warrants a SEMVER major change.

This may involve #110.

[julianrex]

cc @nishant-karajgikar

[1ec5]

We should consider whether this warrants a SEMVER major change.

#60 was a semver-major change in disguise, but I don’t know if adding support for building from source would be. Under which scenarios would someone following the current installation instructions result in a broken build after running carthage update?

[1ec5]

#143 would also point Carthage to this repository but wouldn’t entail building from source. In my opinion, it gets us most of the benefits of building from source without the attendant build system changes.