Fix gradle build dependencies

The 'start' version has missing bits in the pom.xml as well as differences in what
the guide said. After comparing with 'complete', made the fixes and updated the
guide appropriately so it should work top-to-bottom if you use 'start' and really
copy-n-paste your way along.

Add plugin repositories for snapshots

Work in progress, pushing now in order to share with rest of content
team.

Initial runnable app working with Maven and Gradle deps

Ensure the user can execute a `mvn package` after every step, i.e. that
there is never a moment in which the user sees a compilation error in
the IDE or at the command line.