Support for configuring the time until the persistence cookie expires.

Rather than being hardcoded to `24.hours`, the time until the persistence cookie expires is now defined by `GoogleAuthenticatorRails.time_until_expiration`.

With this change, the time can now be overridden in the Rails application by simply doing the following in an initializer:
```ruby
# config/initializers/google_authenticator_rails.rb
GoogleAuthenticatorRails.time_until_expiration = 1.month
```