Added preliminary git support for ctime, mtime and author metadata.
I use GitPython and make sure to follow the log so we can handle renames correctly.
This makes the git support inefficient, and I'm not sure if mercurial handles this anyway
so I might end up revisiting this later
Support metadata embedded in the blog files using MultiMarkdown (in python).
Folding markdown support into the core made this easier, so I got rid of the markdown plugin usage in the templates