Stops relying on global $post var when doing save_post

Instead, the $post object is passed as an argument to the callback.
Merge branch 'hotfix' into development
Changelog for v0.4.2 in README
Merge branch 'development': v0.4.2 – Bugfixes
Merge branch 'master' of github.com:Webbgaraget/wg-meta-box into dev/0.5
Updates changelog
Fixes #21
* Adds generic method to enable sortable admin columns
Updates documentation