Move Pegdown parser to liferay-docs

Hi Rich,

I've moved the code into liferay-docs so we can modify it, test it, use it in liferay-docs for the dev-site article testing, and build its JAR file for use in Knowledge Base.

https://issues.liferay.com/browse/LRDOCS-853

It'll be nice seeing the pegdown conversions locally. :)
Jim