add quotation semantic element to oerpub editor
put quotation link in 'add a new...' menu
refactor quotes to use blockquote node type
center quotation text
fix quotation placeholder text
Revert "center quotation text"

This reverts commit 8a8991ba87967dd4f7ddcc437a18c4f09ae94cb1.

Conflicts:
	src/plugins/oer/quotation/css/quotation-plugin.css
refactor some quotes in the quotation plugin
Merge pull request #33 from oerpub/feature/quotation

adds quotation semantic element