Fully convert to using Gradle instead of Maven for dependency management and building.
This change also includes using Jib again as it works properly for multi-module projects with Gradle.
Some files have been relocate to places that semantically make more sense and work will with a proper build system.
Added GitProperties for handling certain variables that need to be set at build time (for example the version name seen by users) as these were previously set through Maven hackery.
