Move markdown library.

Move markdown library.

This moves markdown into two places:

- The one under samples/ is intended to be a "real" markdown parser.
  Eventually, it will probably live as a third-party library outside of our
  standard lib but still easily accessible.
- The one inside dartdoc will change to become the parser used to parse
  Gilad's forthcoming dartdoc comment syntax.

Committed: https://code.google.com/p/dart/source/detail?r=2606

[dgrove, 2011-12-20 01:27:48]

lgtm