Rearrange block classes for closer spec accuracy.

 - Move both containers and leaves under the 'block' package.
 - Readjust interfaces to use proper Java naming schemes.
 - Remove the excess bullet- and ordered-list specific marker
classes.
 - Add various tweaks for further testing.
Add inline content to the AST.
Add tests for container-level block functionality

 - Include dependencies for JUnit 4 and AssertJ.