Feature/carthage support

[joshrl]

Here's a patch based on your latest release that adds carthage support. This should also fix your travis build (swiftlint is set to prevent builds if line-length > 150 chars).

Note that this remove the post-archive script builds a fat binary - maybe I am missing something, but this seems redundant (there is a script build target that appears to do the same thing and a script build_framework.sh that also appears to do the same thing).

[scana]

Hi @joshrl, thank you for your PR - it's really helpful, especially that I'm not that much of familiar with Carthage.

I've cherry-picked most of the changes from your fork and pushed them to master.

I omitted remove post-archive script commit on purpose as I am still not sure whether it should get removed or not and I prefer not to risk it. I think it is no longer valid with Xcode 10, but I need to verify that without it everything works as it suppose to work for all of our clients.