add a validation error to the current form if a form of a previous step became invalid
allow disabling revalidation of previous steps
added Polish translation
added French translation, contributed by Wolfgang A. Borchardt
added Spanish translation, contributed by Matthias Holder
added Dutch translation
added Russian translation
added Ukrainian translation
added Chinese translation, contributed by 黃沁瑜 (Nancy Huang)
Merge pull request #98 from craue/revalidate-previous-steps

add a validation error to the current form if a form of a previous step became invalid