Run scala:compile goal when just `compile` is requested.

Previously, you would need to request explicitly `scala:compile` in
order to compile Scala files. The `scala:compile` would not be run as part
of standard commands like `package` or `install`.

Now scala goals are properly integrated into standard Maven goals.
Merge branch 'master' of github.com:gkossakowski/scala-sbt-cross-compile