We had some model classes (e.g. MendeleyHighlightBox) not adhering to the NSSecureCoding protocol. Rectified that - by introducing a MendeleySecureObject (<NSSecureCoding>) with MendeleyObject as subclass. Models that don't have an object ID can simply inherit from MendeleySecureObject to ensure they do the encode/decode properly.
Merge branch 'Development'

* Development:
  We had some model classes (e.g. MendeleyHighlightBox) not adhering to the NSSecureCoding protocol. Rectified that - by introducing a MendeleySecureObject (<NSSecureCoding>) with MendeleyObject as subclass. Models that don't have an object ID can simply inherit from MendeleySecureObject to ensure they do the encode/decode properly.
Version 0.9.10: ensure that all model classes adhere to the NSSecureCoding protocol