macOS support added, automated build process, documentation, WIP

[DanBurkhardt]

• C++ adapted for macOS
• Quarternion & Euler measurements verified working on macOS (in Xcode / Swift)
• Automated build process added to mostly automate the compilation tasks for mac
• Dedicated build artifacts and scripts directory for each platform-- I think we should update with Windows (and eventually Linux) build processes respectively.
• Detailed instructions added to README (with disclaimer)
• Fully tested with a working macOS application in Swift(!) (sample project and demo content for that will be posted on the draft PR shortly)

Knowns / issues:

• AirAPI_Windows.cpp will only compile for macOS in this version (most Windows stuff removed or commented)
• I need feedback on my implementation of pthread. It does the job, but it produces a crash upon exit in Swift

This is WIP and may need correction / fixes / stylistic changes. All good, please comment on the draft PR!

(and, uh, maybe if we're gonna keep this going for cross-platform we have to think about the naming, but that is for another time :)

[DanBurkhardt]

closing to re-open as a draft PR instead, see comments and comment in AirAPI_Windows.cpp for more on what feedback I need.