Add `alert(), confirm(), prompt()` implementation

PhantomJS:
  http://phantomjs.org/api/webpage/handler/on-alert.html
  http://phantomjs.org/api/webpage/handler/on-prompt.html
  http://phantomjs.org/api/webpage/handler/on-confirm.html

Capybara::Driver::Base#accept_modal:
  https://github.com/jnicklas/capybara/blob/2.4.1/lib/capybara/driver/base.rb#L95-L107
Capybara::Driver::Base#dismiss_modal:
  https://github.com/jnicklas/capybara/blob/2.4.1/lib/capybara/driver/base.rb#L109-L120