[racket-dev] Two pull requests (Markdown render for Scribble, and XREPL , log udpate)

I submitted two pull requests:

https://github.com/plt/racket/pull/187
Add a Markdown rendering mode to Scribble.

https://github.com/plt/racket/pull/188
Update xrepl for 5.3.1's enhanced `make-log-receiver'.

This is my first time submitting a pull request to Racket. I tried to
make it correct and convenient: Each pull request uses a dedicated
topic branch with a single commit. If there are any questions or
problems, of course please let me know.


P.S. Adding a Markdown render to Scribble is something I've wanted to
do for awhile. What motivated me to finally do it was thinking about
how I could provide documentation on Github with Planet2.

For example, the markdown at https://gist.github.com/4230519 is
generated from the same .scrbl source as the HTML at
http://planet.racket-lang.org/package-source/gh/aws.plt/1/4/planet-docs/manual/index.html.