Add Jetpack Open Graph tag support for Guest Authors

fix #146
Fix author feed link
Make these methods explicitly public
Some improvements to the PHPdoc
Make this method explicitly public
More PHPdoc for the methods
Merge pull request #188 from DFMdata/docs

Docs improvements
Remove unused `get_post_id()` method
Merge pull request #189 from Automattic/rm-methods

Remove unused `get_post_id()` method
Increase minimum required WordPress version to 3.7

We're using core's unit testing framework and it doesn't run reliably below 3.7. It doesn't make sense to support old versions of WordPress indefinitely either.
Latest is 3.8 right now, so this is fine
Merge pull request #190 from Automattic/bump-required

Increase minimum required WordPress version to 3.7
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