v0.4

This release adds experimental support for encryption of database notebooks using the cryptography library.

• Adds new configurable crypto attributes to PostgresContentsManager
  and PostgresCheckpoints. crypto objects must provide encrypt
  and decrypt methods, which will be called on data being written to
  and read from the database, respectively.
• The default crypto implementation is NoEncryption, which returns
  its inputs unchanged.
• The built-in crypto implementation is FernetEncryption, which
  uses cryptography.fernet to implement symmetric-key encryption of
  all notebooks and files in the database.
• Adds re-encryption utilities to pgcontents.utils.sync.