Replace api.github.com for mark-down conversion.

Using api.github.com for mark-down rendering limits off-line use and cause trouble with help is excessively used on a shared IP due to api quota rules.

This patch includes Christopher Jeffrey's Marked mark-down compiler (https://github.com/chjj/marked) and replaces the github api call.

From: Niklas Nielsen <nik@qni.dk>
Review: https://reviews.apache.org/r/14940