[PR] Update Spine build process to be available for multiple deployed versions

This is the first set of changes toward a build process that supports multiple live versions in production. Our primary development branch is now **develop** instead of **master**. We will use **master** for tagging stable, semantically versioned releases.

* Build only in `build/`, do not include a version number
* Move `demo.html` to the `build/` directory, as it can now be directly accessed.
* Force HTTPS in some areas that can be to prevent failed relative `file://` URLS.
* Use `develop` as the `build_version` when working in the develop branch to help build tests in proper location.

Additional work on its way.