Add support for YAML metadata/description files.

[DaloLorn]

In the years since first designing the Launchpad, I've grown quite fond of YAML. Much less verbose, and feature-richer, than JSON, I can hardly resist the urge to use it as a replacement for JSON wherever I can.

• Replace Gson dependency with something YAML-enabled. (Not sure what yet, but I'll figure it out.)
• Backwards-compatibility: Add functionality to the OpenSR Toolset to autogenerate a branch description or metadata JSON whenever its corresponding YAML file is saved. (To be tracked in Autosync Launchpad YAMLs to corresponding JSONs OpenStarRuler-Toolset#1.)
  • Without this feature, migrating to YAML would break that repo's compatibility with older versions of the Launchpad. 😦