Rework ListField and DictField

[lafrech]

Closes #105.
Fixes #78.

• DictField and ListField don't default to empty Dict/List (see Empty DictField in ListField #105).
• Serialize empty Dict/List as empty rather than missing (see Empty DictField in ListField #105).
• Fix passing a default value to a DictField/ListField as a raw Python
  dict/list (see Issue passing a missing value to a ListField #78).

[coveralls]

Coverage increased (+0.02%) to 96.245% when pulling b5b7422 on rework_list_dict into 3b33dc1 on master.

[coveralls]

Coverage increased (+0.02%) to 96.245% when pulling da54b09 on rework_list_dict into 3b33dc1 on master.