Add Swift Package Manager support

[pchmelar]

:pencil: Description

• This PR adds Swift Package Manager support

:bulb: What’s new?

• SwiftSocket can be integrated via Swift Package Manager
• iOS deployment target bumped to 9.0 in order to mitigate warning in newer Xcodes
• Add instructions to README.md

:no_mouth: What’s missing?

• Add 2.2.0 tag after merge

:books: References

• https://stackoverflow.com/questions/49779543/swift-package-manager-c-interop-non-system-libraries

[gabrieloc]

Any chance we can get this merged?