Added Carthage support and upgraded to Swift 4

[gapl]

Description

• Upgraded to Swift 4.
• Added an Xcode project with a shared build scheme so Carthage is able to build a dynamic framework.
• Updated README.md to reflect new installation option.
• Tested everything from my personal fork.

After merge

Please tag this as a stable release on GitHub so Carthage will be able to find it. Carthage determines which versions of framework are available by searching through the tags published on the repository, and trying to interpret each tag name as a semantic version. For example, in the tag v1.2, the semantic version is 1.2.0.

[huebnerob]

Carthage support looks great 👍. However, the sample project doesn't build anymore. We'll need to fix that and also verify that CocoaPods integration is still all good (I didn't test this yet, could be a non-issue).

[gapl]

Nice catch, will fix that over the weekend and I'll also verify that Cocoapods integration still works.

[gapl]

@huebnerob I was looking into this issue over the weekend and I was having trouble getting the tests to pass, even by checking out the vanilla master branch. Seems like with Xcode 9, the tests are not passing anymore (the project doesn't even build to be precise).

The test Example project needs to be built against Swift 4.0 and have its 3rd party dependencies updated. Unfortunately, I don't have much time to work on this. If someone is willing to jump in and update this PR, I'd be more than happy. Otherwise, I can close this PR and keep working from my fork 😕

[huebnerob]

Ah, alright, sorry this is more complex than expected. I will have some time to look into it tomorrow and I will report back.

[huebnerob]

@gapl I've opened #9 to support Xcode 9 and Swift 4 in the sample project. I'm still working on getting BuddyBuild to stop choking on the tests, after which I can merge that PR, but everything seems to be good locally now. Would merging this xcode-9 branch into yours unblock you?

[gapl]

@huebnerob Thank you for taking the time for this one! Don't worry about unblocking me, I've since worked off of my own fork, but wanted to contribute back with this PR. I'll rebase this PR onto #9 and go from there.

Sidenote: I'll be afk for a few days so feel free to take over if needed.

[huebnerob]

@gapl Great, thanks for your contributions! I'll continue to work on getting the xcode-9 branch to 100% for now.