REL: Release 0.5.1.

- Adds a new `fetch` command to the `pgcontents` CLI for fetching a
  single notebook.
- Adds utilities for offline processing of notebooks from the pgcontents
  database.
- Adds utilities for encrypting and decrypting notebooks for many users.
- Pegs `notebook` dependency to < 5.0.

setup.py

@@ -23,7 +23,7 @@ def main():
 
     setup(
         name='pgcontents',
-        version='0.5.0',
+        version='0.5.1',
         description="A Postgres-backed ContentsManager for IPython/Jupyter.",
         long_description=long_description,
         author="Scott Sanderson",