[Po4a-devel][patch] Take 2: Making Html.pm (a little) better

Ok, here is a much bigger patch: Thanks to Nekral for pointing out 2 problems: - Those two spaces around the pushline() call where wrong, but so was what I did. After further thinking, it's actually obvious that you _cannot_ touch leading and trailing spaces. They are now conserved, and all we do is remove multiple spaces. - The title/alt attribute translation was indeed wrong. Now the <img> tag is rewritten entirely. Additional things: - tokens with no content shall be translated no more (things like <b>hello<b>, <i>world</i> would generated a msgid ", ") - Test suite added. Thanks to Denis for making me do this (made me find potential problems) and Jordi for pointing po4a-normalize. Enjoy! Y. - damn, soon I'll run out of excuses for not translating the site.