Update Python-markdown module to version 2.4

The old markdown parser was broken for me in ST3. Dropping in version 2.4 fixed all problems.

See: https://github.com/waylan/Python-Markdown/blob/master/docs/release-2.4.txt

- Adds official Smartypants extension (smarty.py)
- Remove mdx_smartypants.py as it's no longer needed