Update Scala documentation

Summary:
- Refer to Java Guide (related to travis-ci/travis-build#130)
- Better explain how JVM and SBT settings can be customized (related to
  travis-ci/travis-build#122)
- Add more Scala projects in Examples section
- Brand 'sbt' everywhere

@joshk These changes already correspond to current production (maybe except travis-ci/travis-build#130 if not rolled out already) and the documentation will remain totally valid after upcoming VM update (see travis-ci/travis-cookbooks#220). Note that I created the feature branch in main repository, so that other team members can update it (e.g. english quality, ...).