Add Scalastyle plugin

For the beginning the only rules that are activated are rules that
check for the existence of trailing whitespaces and tabs.

The Scalastyle plugin runs at the very beginning of the build process,
it is the first call executed in the build-all.sh script.

Whenever some style errors are found the complete build is aborted,
which means that all style errors needs to be fixed before new code
can be merged.

To run the plugin manually simply type in:

  mvn scalastyle:check