Hardening/changelog update

Implements #269 

(Sorry, I missed the 269 reference in the branch name)
Changelog is ordered in chronological order. It is a bit weird that 0.5.1 comes after 0.4.2 (as 0.5.1 > 0.4.2). However, probably is only an "aesthetic" problem and we can live with it.

What do you think?