POM cleanups

  * Scoped some dependencies more restrictively (e.g. mockito into the `test` scope)
  * `reef-examples` now also creates `target/reef-examples-0.1-SNAPSHOT-shaded.jar` which contains it and all its dependencies in a shaded JAR. You can e.g. run the HelloREEF example on the local runtime via: `java -cp reef-examples-0.1-SNAPSHOT-shaded.jar reef-examples-0.1-SNAPSHOT-shaded.jar`
  * `reef-runtime-yarn` no longer creates the `-with-dependencies` JAR as it is no longer needed.