https://issues.liferay.com/browse/LRDOCS-1312

https://issues.liferay.com/browse/LRDOCS-1312

Added convert.sh script to call Liferay's Markdown converter. Added convert.bat script for Windows and updated usage message of convert.sh

@jhinkey Hey Jim, could you or someone from NC review this .bat script? Please refactor as you see fit; I haven't done much batch scripting.