Add Periodical, PublicationIssue, PublicationVolume

Add the types and properties 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 periodical and article examples

Having examples with 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>