Add Periodical, PublicationIssue, PublicationVolume

Add the types, properties, and examples per
http://www.w3.org/wiki/WebSchemas/Periodicals,_Articles_and_Multi-volume_Work
along with the modifications for existing types and properties, per discussion
on public-vocabs.

Signed-off-by: Dan Scott <dan@coffeecode.net>
Add article citation example and example descriptions

public-vocabs wanted an article citation example, so add that.

Having descriptions that provide a rationale for the existence of the
example seems likely to be helpful to users of the schema.org docs.

Signed-off-by: Dan Scott <dan@coffeecode.net>
Add Periodical types and changes to docs/full.html

Signed-off-by: Dan Scott <dan@coffeecode.net>
Merge branch 'periodical' into dbs_fixes

Signed-off-by: Dan Scott <dan@coffeecode.net>

Conflicts:
	data/examples.txt