Convert to Unity Package

[idbrii]

Not sure if you're interested, but this makes installation easier -- just add to your manifest.

Required moving files around, adding asmdef, adding package.json, and adding meta files.

This PR changes the install directory, so it includes #1. That PR should be addressed first anyway since it's more fundamental.

I also removed the png which you might not like, but I'd guess you're moved on to ReferenceFinder and don't plan to maintain this project any longer. Fine by me! Thanks for releasing the code!