Add #skip_confirmation_notification! to Confirmable

This pull request adds an instance method, `#skip_confirmation_notification!`, which sets a flag on a confirmable object that will skip the `send_on_create_confirmation_instructions` callback, allowing you to manually `send_confirmation_instructions` at your leisure.

I have an app in which I want to create a confirmable user record as part of a transaction, and only send the confirmation e-mail if the entire transaction succeeds. `#skip_confirmation!` is not the correct solution for my use case, because it does not *only* skip sending the confirmation e-mail, but it also immediately confirms the user account.

[This stackoverflow post](http://stackoverflow.com/questions/4343454/delay-and-or-resend-devises-confirmation-email-for-manually-created-users) and devise issue #2198 also suggest that there are other use cases in which this behavior would be desirable.

This is my first attempt at contributing to an open source project, and I'm pretty new to software development generally, so I would appreciate any feedback on ways this pull request could be improved.