Added special fields 'Printed' and 'Read status'

Added special fields 'Printed' and 'Read status', thereby implementing (at least partially):
+ Feature #762 Printed attribute
+ Feature #692 Marking of new entries and often read entries
+ Feature #685 Highlight new entries that were "forgotten"
+ Feature #602 could add the mark as read feature
+ Feature #225 read? checkbox

Printed is just a boolean toggle and for the read field currently the values 'Skimmed' and 'Read' are possible (if you have any suggestions for further possibilities, I'm happy to implement them). The icons are taken from http://www.softicons.com/system-icons/crystal-project-icons-by-everaldo-coelho and should probably be replaced with icons from the same collection as the other currently used ones.