Merge pull request #2 from britishtea/master

Making Shield work with ORMs other than Sequel and Ohm.
Make PBKDF2 work.
Merge branch 'vendor-pbkdf2' into refreshment

Conflicts:
	lib/shield.rb
Works with PBKDF2 gem again, with just a minor patch.

Latest resolution: release a fork of PBKDF2 that has the fixes
and also has cleaner code.