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.

I based the syntax of this change log off of a project I started here: http://keepachangelog.com

I think it's great that you had a change log maintained in the Wiki for so long, but wikis do not travel along with the project source code, which can easily see them become out of sync (which happened here). It seems 
much less likely to happen when the change log is a metadata file alongside the codebase.