Release of 0.5.7

This release includes metadata for next and previous documents

• ENH: add next and prev document metadata to notebook meta #329 (mmcky)

The metadata is provided at the notebook level in nb.metadata and is in the following format:

"next_doc": {
   "link": str(docname),
   "title": str(title)
  },

with metadata keys: next_doc and prev_doc.

If a next/previous document does not exist there is no metadata included for that item.