Replace email regex with EmailValidator gem

* Use Rails 3 `validates` method.
* Use Ruby 1.9 hash syntax.
Fix error in Blowfish, SHA1 password strategies

* Bug report:
  https://github.com/thoughtbot/clearance/issues/222
* Remove duplication in spec suite.
* Extract `fake_model_with_password_strategy` spec helper method.
Bump bersion to 1.0.0-rc5