Allow registered users to sign in
Allow signed in users to sign out
Support user lookup field configuration in forms

Sign in and Sign up forms use the configuration for the user lookup
field. Moved the form partials back into the views that were using them.
Rename `Config` class to `Configuration`.

It's still exposed on `Clearance` as `config` to be consistent with
other gems, but there's no need to be lazy with the class name.
Set rake to run rspec as the default task
Extract Clearance::BackDoor to file
Add README
User can request a password reset

This change introduces a `PasswordReset` model to store the token
generated for a user resetting a password. It also re-introduces
ClearanceMailer which sends the reset notice to the user.

The install generator was updated to copy a migration that creates the
`password_resets` table (though this name can be derived from an
optional argument to the generator).

Configuration options were introduced to drive the password reset
process:

* `mailer_sender` is the from address for the mailer.
* `password_reset_class` is the class to use for password resets.
* `password_reset_delivery_method` is a lambda used in the
  `PasswordResetsController` to deliver the mail and is intended to be
  overridden in order to introduce background mail delivery.
Add travis configuration
Extract dummy app setup from spec_helper