Update handling of e-mail confirmation

[danschultzer]

A bunch of refactoring after #255, made the tests and mocks more readable, and fixed the following logic:

• Updated PowEmailConfirmation.Ecto.Schema.changeset/3 so;
  • when :email is identical to :unconfirmed_email it won't generate new :email_confirmation_token
  • when :email is identical to the persisted :email value both :email_confirmation_token and :unconfirmed_email will be set to nil
  • when there is no :email value in the params nothing happens
• Updated PowEmailConfirmation.Ecto.Schema.confirm_email_changeset/1 so now :email_confirmation_token is set to nil