Make @user.setup_activation a public method

Curretly @user.setup_activation is a protected method of User class,
but supposedly it is useful to make the method callable whenever and
from wherever you want.
For example, you can require users to re-activate their accounts when
they change registered emails.
Update README.md
Update CHANGELOG.md
Merge pull request #576 from iTakeshi/publicate_setup_activation

Make @user.setup_activation a public method