Feature/swift pm support

[chrisob55]

What did you change and why?

• Added support for Swift Package Manager
• Modified the folder structure to make it Swift PM compatible
• Deleted Sources/TrueTime.h and Sources/TrueTime.m and replaced the declarations in Swift

Potential risks introduced?

What tests were performed (include steps)?

• Tested within an app that previously used TrueTime via Carthage

Checklist

• [ ] Unit/UI tests have been written (if necessary)
• [x] Manually tested

[glizik]

I would love to see SPM support.

[chrisob55]

I would love to see SPM support.

I have this which I will be keeping forever (or until the main repo adds SwiftPM support)