Add Jetpack Open Graph tag support for Guest Authors

fix #146
Check if post type supports co-authors
Change method to Public and add proper php comment block
Move the post type check to a more logical place
Merge branch 'master' into fix-146

Conflicts:
	co-authors-plus.php
Mixing branches, yes, but this method makes more sense in the Guest Author class
Props for #187
Merge pull request #146 from Automattic/fix-146

Add Jetpack Open Graph tag support for Guest Authors