labelled 2.5.0

• new .keep_value_labels argument for recode.haven_labelled()
• new .combine_value_labels argument for recode.haven_labelled() (#61)
• new drop_unused_value_labels() method
• an additional .labels argument for set_value_labels()
• user_na_to_na argument has been added to to_character.haven_labelled()
• %>% is now imported from dplyr
• a cheatsheet has been added (#47)
• internal documentation is now using roxygen2 markdown support