Add migration for new `remember_token` field.

Also revoked useless field from 22c7aaf9fb4915.
Add table scope wrapper.
Change `remember_token`'s type.

Use nullable text.

Ref: https://gist.github.com/denvers/10828426
Merge pull request #27 from bcho/hotfix/remember-user-field

Add migration for new `remember_token` field.