Add PKCS5 password strategy.

This is the first of two pull requests (hopefully).  If Blowfish is going away, PKCS5 is a good alternative from what I know.  It's designed for password encryption.  It still requires the salt column so hopefully that's not the goal of deprecating Blowfish (or possibly SHA1).

The appraisal steps from `CONTRIBUTING.md` and the complete test suite failed for me on master/v1.5.0 without my changes.  I'm not sure of the status of appraisal checks for pull requests.  Maybe I can take a look at the rspec and cucumber suite ...
Prefer single-quoted strings when you don't need string interpolation or special symbols.