Refs #415. Converted block spacing elements to using editor-only class.
Refs 415. Updated structured headings plugin to use the editor-only class on the numbering spans.
Refs #415. Megre #414 and fix deprecated html() function so it still works in all use cases.
Merge branch 'issue_414' into issue_415
Refs #415. Updated test to use _html() instead of html().
Refs #415. Readded BLOCK_ELEMENT_SPACER_CLASS.
Refs #415. Added test for setting and getting the editor's html using the html() function.
Merge pull request #415 from FriedRice/issue_415

Closes #415. Convert previously implemented editor-only elements to using the newly added editor-only class