Implement Logging Out

If users can login, we’ll also want to allow them to logout. Implement a controller method that will log a user out when they visit it. This will most likely involve deleting some content from the session hash. Use the following user story to help guide your implementation:

As a registered user
When I am logged in and visiting the users index
And I click on a link to 'Log Out'
Then I am on the users index
And I see a link to 'Log In'
And I do not see a link to 'Log Out'

Next to Iteration 4