Add Dropbox's zxcvbn library for realistic password strength estimation.

Upstream: https://github.com/lowe/zxcvbn. MIT License.
Modified for WordPress with a rot13 of the included word dictionaries, for PG-ness.

props duck_.
see #21737.


git-svn-id: https://develop.svn.wordpress.org/trunk@25156 602fd350-edb4-49c9-b593-d223f7449a82
Use Dropbox's zxcvbn library for our password meter.

The library was added in [25156].

props duck_.
see #21737.


git-svn-id: https://develop.svn.wordpress.org/trunk@25157 602fd350-edb4-49c9-b593-d223f7449a82