ui: cards now have a thick color bar on the left
ui: cards now have a thick color bar on the left

This can be used to provide status indicators for cards (e.g. red bar
for overdue)

CS2103AUG2016-T11-C4/main,CS2103AUG2016-T11-C4/main,nightism/addressbook-level4,snehasp13/main,snehasp13/main,nightism/addressbook-level4