[4.2] Add password helper to the schema Blueprint

Like the existing `rememberToken()` helper, this makes it easy to create a 60 character string column for a hashed password (I actually forget on every new project that the hash is 60 characters and have to go look it up in an earlier project to check).

It will default to a column named password, but allows you to pick your own name for the column if you prefer.
And there can't be an empty password. Because it's always hash there always be a value even for empty password. I hope that other checks in PHP are in place to prevent user from logging-in with an empty password in if matches hashed version in db.