Added methods for mouse_down, mouse_move and mouse_up in the driver
Add `alert()` implementation

refs:

- http://phantomjs.org/api/webpage/handler/on-alert.html
- Capybara::Driver::Base#accept_modal
    - https://github.com/jnicklas/capybara/blob/2.4.1/lib/capybara/driver/base.rb#L95-L107
Add `confirm()` implementation

refs:

- http://phantomjs.org/api/webpage/handler/on-confirm.html
- Capybara::Driver::Base#dismiss_modal
    - https://github.com/jnicklas/capybara/blob/2.4.1/lib/capybara/driver/base.rb#L109-L120
Add `prompt()` implementation

refs:

- http://phantomjs.org/api/webpage/handler/on-prompt.html
Update dependencies for Capybara

- Capybara 2.4
    - Modal APIs (alerm/confirm/prompt)
Fix CHANGELOG for content on modal APIs.
Merge remote-tracking branch 'michiels/implement-mouse-events'