Wikiprova.md: explicit list at the end

Also, leave a blank line before the list, to make the text
a bit more readable.
Wikiprova.md: wrap text before column 80

This is the de-facto standard in the UNIX world, because ancient
terminals were 80x24 or 80x25.
Wikiprova.md: center the images

It is easthetically more pleasing to have images centered.
Revert "Wikiprova.md: center the images"

This reverts commit dc8cd8639d..., basically because there is no
tangible effect. Doh.
Merge pull request #1 from bassosimone/master

Aesthetical tweaks