JSONList: added parameter 'unique_sorted': stored value is a list with sorted uniquified terms
db.Model: install method 'display_value' to retrieve human-readable value of a field; useful for fields having a 'choices' dict in their info dict.
display_value: handle list of items
fix missing param in call of supermethod
made Mutable elements support pickling
see http://docs.sqlalchemy.org/en/rel_0_8/orm/extensions/mutable.html#supporting-pickling
ListWidget: ensure to use label instead of raw value when field has choices selection