Format code using Scalariform Maven plugin

Scalariform setup follows as closely as possible the currently used formatting.

It is not, however, compliant with [Scala Style guide](http://docs.scala-lang.org/style/) (if we don't consider that we are using tab indentation instead of 2-space indentation), as [double indent class declaration](https://github.com/mdr/scalariform#doubleindentclassdeclaration) is disabled.