Make zxcvbn stateless w.r.t user_inputs

Changed scoring to be stateless with respect to the `user_inputs` parameter. I've also added a test to cover this specific case.

@lowe please have a look and let me know how amenable you are to this change. If necessary I can make this change backwards compatible so as to not affect existing clients, for example by adding another param that callers have to provide to opt in to the new, stateless, behaviour.

This fixes issue #31 in dropbox/zxcvbn.