Add Unity Package Manager support

[mikz]

This PR adds package.json to add support for the Unity's Package Manager.

It makes minimal changes to the repository structure, so it does not follow Unity's rules, but it still works great.
The only change to the structure is renaming Examples folder to Examples~, so it is not imported as part of the package, but rather optionally as a sample.

You can try this by adding a package by Git URL https://github.com/timewarpinc/SuperTiled2Unity.git?path=SuperTiled2Unity/Assets#upm-pr.
After merging to your branch it will be possible to just do https://github.com/Seanba/SuperTiled2Unity.git?path=SuperTiled2Unity/Assets.

I'd suggest to update the package definition to link to your donation page.

Cheers!

[Seanba]

Hi there. I've long been thinking about support for Package Manager. I've got some tasks to prioritize for now but I'll come back to this when I can. I appreciate you getting the ball rolling.