show_values: only allow :all, :none, :paged, :bulk, :print.
Added show_values_in_mode? helper.

Closes #108

LGTM given by: @marten

Squashed commit of the following:

commit 515c6f4ef424262bc87cd3dcfefd28b6230cf06a
    Fixed and cleaned up _item_question_number_spec
commit faa75c9a13c3587b2968d65972e5b7e8df220787
    Made all view use the new show_values_in_mode? method. Fixed paged/checkbox that was showing when the value was :print.
commit 6e0324fbf73ba09ebb16320a9c95aa2e639b09d1
    Added show_values: :print option to question. Added show_values_in_mode? method to question
commit 31eebce65a0642c46c660b92d8d957bf235de544
    allow only :all, :none, :paged and :bulk for question#show_values