2.1.0: Traversable support

• Merged PR #7: Add support for all traversables to Json::write()
  (@thekid)
• Merged PR #8: Add reset() operation. Enable calling the elements(),
  pairs() and read() methids again after explicitly resetting the
  stream. This operation may raise an exception if the input is not
  seekable, e.g. if the underlying stream is socket I/O.
  (@thekid)