use hd in sidebars and poems. wrap inline note content in paragraphs.

 * added validation rule nordic268: heading levels must be nested correctly; h1, h2, h3 etc.
 * use hd for headlines in sidebars and poems, fixes #251
 * added tests for hd element in sidebars and poems
 * wrap inline content in notes in p elements, fixes #252
Merge pull request #253 from nlbdev/issue-251-252

use hd in sidebars and poems. wrap inline note content in paragraphs.