Add Jetpack Open Graph tag support for Guest Authors

fix #146
Fix author feed link
Check if post type supports co-authors
Change method to Public and add proper php comment block
Make method public and add proper comment block
Limit this to guest authors only
Fix error in last commit, used array rather than object referencing
Merge pull request #186 from hibernation/author-feed-link

Fix author feed link
Props for #186
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