CHANGELOG.md: improve formatting, fix typos

Relates to #663
See http://keepachangelog.com/en/0.3.0/
See http://sebsauvage.net/wiki/doku.php?id=php:shaarli:history

Modifications:
- keep 2 newlines between each release section
- keep reasonable line lengths (< 90 characters)
- remove extra final dots (only kept when there are several sentences)
- fix typos
- reword/rewrite/curate legacy changelog entries

Signed-off-by: VirtualTam <virtualtam@flibidi.net>