Global checkin implemented. Has the option to analyse the files and it will return information about what has been checkedin and what references have been found.
Tidied up references. References are now only invalid if they can't be found in the system. Working file and published file references are considered valid for flexibilities sake. The UI will still only allow published files to be referenced though.
References are now editable. Assets are now completely uneditable whilst checked out. Closes #28