Added how to translate instructions to the FAQ

I have added how to translate the wizards text to the FAQ so it is easier for other people to find how to do it. Also, mentioning that it could be good to send their translations to the project.
Merge pull request #24 from PhoneixS/patch-1

Added how to translate instructions to the FAQ
Disabled Finish button when going backwards.
Added cache to all buttons.

Now when you go back and forward the status of buttons (previous, next,
cancel and finish buttons) are cached except in pages which are
transients.

Also have change the initialization code from the declaration to the
constructor so we don't have initialization code in both places.