Cleaner support PKCS#12 without passphrase.

The upgrade from 0.13 -> 0.14 had a small backwards incompatible API change
that, in 0.14, requires `load_pkcs12` to pass in a passphrase. This change
makes the API backwards compatible by setting a default value for the
passphrase argument.

Add test cases and change log entry for changes.
Fix too-long lines and fix repeated spelling error.
Merge pull request #108 from pyca/pkcs12-without-passphrase

Re-enable support for loading PKCS12 containers that have no passphrase or an empty passphrase and let this be the default when no passphrase is passed to `load_pkcs12`.