Create CHANGELOG.md

Based on the recently not updated https://github.com/wvanbergen/chunky_png/wiki/Changelog
and the version release info on http://rubygems.org/gems/chunky_png/versions.

Missing notes for the most recent releases but here's hope someone more familiar with the project
will find the time to fill these in.
Convert CHANGELOG to RDoc
Fill out empty sections of CHANGELOG
Merge branch 'olivierlacan-patch-1'