Version 0.1.3

Added two new features - serialization objects and object key serialization:

• serialization objects allow one to "specialize" generic read()/write() pickler
  methods for some specific type for convenience and type safety;
• object key serialization makes it possible to serialize maps with arbitrary keys
  as backend objects (which currently is possible only for string keys) provided that
  there is a special serializer for key type is present in implicit scope. It is also now
  possible to disallow the default behavior when maps are serialized as arrays of arrays
  and only enable it explicitly for specific key types.