v0.2.1

• changed type checking keyword argument names: opt -> or_none
  and nodups -> unique
• improved error messages for constructing Structs
• significant updates to readme and examples
• using opt=True on TypedField no longer implies that None is
  the default value
• made mixin version of checktype() and checktype_seq()
• added check() and normalize() hooks to TypedField
• accessing fields descriptors from classes is now permissible
• added support for default values in general, and optional values
  for type-checked fields
• fixed __repr__() on recursive Structs