Foundation with all block elements in place using OO principles

This foundation should parse all block-level Markdown elements. It is coded
using a more object-oriented approache with a bunch of small types that can
parse a specific Markdown block.

Remaining: Implementing the inline Markdown syntax and proper html escaping
Reorganizing files
Refactoring and cleaning up code
Fixing individual tests so they'll run individually
New type to encode inline Markdown markup mostly complete

Remaining:
  - Still need to create code for reference links / images
  - Wiring up the new type to the existing types (integration tests???)
Hooking in markdown_inlineText
Allowing for Atx headers to close themselves