Replace Maruku with Kramdown as Default Markdown Processor

After giving this more thought, I think the final decision should be about compatibility and the best user experience. Kramdown has syntax most similar to Maruku (for non-standard features, that is), and it has [stellar docs](http://kramdown.gettalong.org/), which makes getting started with it and migrating to it from Maruku wonderfully easy.

Fixes #1752.