v1.0.0-rc.2

Added

• Add a preserve_dict parameter to ItemCollection.from_dict and set it to False when
  using ItemCollection.from_file.
  (#468)
• StacIO.json_dumps and StacIO.json_loads methods for JSON
  serialization/deserialization. These were "private" methods, but are now "public" and
  documented (#471)

Changed

• pystac.stac_io.DuplicateObjectKeyError moved to pystac.DuplicateObjectKeyError
  (#471)