Bump version to accommodate use of 'Error' vs. throwing objects.
Use empty password buffer if password is not given.
PKCS#12 password fixes.

- Pass null/undefined password through to key derivation function
  which now properly differentiates between the empty string
  password and no actual password.
- Use the empty string password to decrypt safe contents if no
  p12 password is given and encrypted contents are found.
Release 0.6.0.
Start 0.6.1.