Support of Swift Package Manager

[TwoDollarsEsq]

Adds Package.swift file which enables usage with SPM and closes #60.
Note, that the most correct usage with SPM will be possible only when there is new release that includes Package.swift. Until then, dependency can be added by branch or commit hash which is not allowed in published packages.

[jimisaacs]

@adamshin Please merge this

[TwoDollarsEsq]

@jimisaacs, if you need this lib asap you can use my fork. It is going to be there for a while. At least until this PR is merged. Or you can create your own fork from mine.

[jimisaacs]

Thanks, already am, just thought I’d add another voice to this pr

[adamshin]

Thanks for the reminder on this. I'm not super familiar with Swift package manager — now that this is merged, is there anything else that needs to happen to make it available as a package?

[jimisaacs]

Nope, that should be it, thank you @adamshin !

[jimisaacs]

@adamshin well, a best practice is adding a version tag, so might want to do that after this merge.

[jimisaacs]

Something to note, with Xcode 11, and with a Package.swift, you no longer need to commit the Xcode project to git! Xcode can open the directory containing the Package.swift, and auto generates the project inside the hidden directory .swiftpm. I'm still looking for best practices for working with that directory and the workspace/project inside it, but I'm like 90% sure it can be ignored as well.

[jimisaacs]

I think it should go without saying (so forgive me that I say it anyway), but don't delete your project until trying out what I described above.

[TwoDollarsEsq]

@jimisaacs, I had some time experimenting with SPM and I can say that you’re absolutely right. Since version 11 Xcode indeed creates .swiftpm directory for packages. Even more, you do no need to work with what’s inside it ‘cause Xcode can open packages natively. Just try to open Package.swift file with Xcode.
With this setup you still can run tests and have auto completion. Moreover Xcode helps troubleshooting issues also.

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