Add 'contributor', 'actor', 'director' (more?) as expected properties on Event

[danbri]

Generalizing from requirements articulated in #445 and #503

Of the various people-valued properties attached to CreativeWork, the following seem useful for describing many events. All of them will need minor and fairly modest textual definition changes to support their use with Event.

• http://schema.org/contributor
• http://schema.org/actor
• http://schema.org/translator
• http://schema.org/director
• http://schema.org/composer

[danbri]

http://webschemas.org/docs/releases.html#g1092