Add common passwords from xato.net

Mark Burnett publishes lists of top passwords occasionally. This PR adds
the top 500 and top 1024 passwords, as of 2011-06-20, linked from this
blog post:

http://xato.net/passwords/more-top-worst-passwords/

He also does a fair bit of frequency analysis there.

The 1024 list should probably used instead of the original
unix_password.txt file. unix_password.txt was added on 2010 from an
unknown source (and since edited occasionally to add known good default
passwords). Pulling those changes into this new list, and then overwriting unix_passwords.txt with this, seems like a helpful thing to do, but let's
keep both lists around for a little while.

As far as I can tell, there are no special licensing terms for these lists.