force UTF-8 encoding for the sake of consistency around all the world

@aL3xa: the special Windows locales can really screw up the exported docs that are to be in UTF-8 by default. So if you create an UTF-8 encoded tpl and run "rapport.html" on that, you'd get all screwed up chars instead of the nice accented Hungarian characters :)

That's not good. I'd rather force UTF-8 usage on all platforms for `rapport`. What you think? We can mention that in the docs...