Allow All Instances of Sharing Meta to Save

Sharedaddy registers its meta box for all public CPTs by default (line
25), but only saves the option in the meta box for posts and pages. The
`sharing_meta_box_save()` function needs to save the meta box's
information for all CPTs to which the meta box is tied.
Merge pull request #650 from ericmann/master

Allow All Instances of Sharing Meta to Save