Re-add the Fieldmanager_Post class.

Hey guys, I noticed that in this commit the Fieldmanager_Post class file was deleted:

https://github.com/alleyinteractive/wordpress-fieldmanager/commit/6c3ec9e3baf61e4ed8e02a312b91228993de0a25

I'm not sure if that was purposeful - if not, this pull request should restore it. If it is purposeful, could you advise on what class/function we should switch use to?