[TASK] Add the ability to modify the class of the current language

A sensible default was provided to not break older versions.
Merge pull request #422 from cedricziel/allowOverridingOfCurrentClass

[TASK] Add the ability to modify the class of the current language
[BUGFIX] Languages argument can be left empty

In that case, the v:page.language ViewHelper will simply return the current languages uid.
Merge pull request #423 from cedricziel/noLangArg

[BUGFIX] Languages argument can be left empty