Add Smarty extension and use it for building docs.

This pull request adds Smarty extension, as discussed in #12. Also, it makes use of that extension when building Python-Markdown's own docs.

The diff of the docs before and after using Smarty: https://gist.github.com/mitya57/6086532.

One cosmetic issue is that `headerid` extension creates ids like [`whatwzxhzdk1s-new-in-python-markdown-20`](https://gist.github.com/mitya57/6086532#file-gistfile1-diff-L756), but links work anyway so that shouldn't be an issue (or can you fix the `headerid` extension instead?)