Fixes #20
* Removes bug where not all admin column values were printed in case of conflicting field slugs
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
Fixes #23
* Adds required option
Merge branch 'dev/0.5' into development
Merge branch 'development'