I think nanoc is worth a try, it has everything you specified, even if it is not the best rating on a ruby instrument, it is actively developing and very customizable.
nanoc is a tool that runs on your local computer and compiles documents written in formats such as Markdown, Textile, Haml… into a static web site consisting of simple HTML files, ready for uploading to any web server.
and it's true :) I have been using it for some time not specifically for the blog, but it also has helpers for this ...
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