Refactor Auth controller login/logout methods.

Rename several methods:

- loggedIn -> setLoggedIn
- loggedOut -> setLoggedOut
  For setting the state of login form.

- getSession -> retreveSession
  "get" is a bad start for method that doesn't return a value.

- showLogin -> showLoginForm
  Previously it was hard to distinguish it from showLoggedIn.

- loggedIn -> isLoggedIn
  Now the method for checking logged in status is named the same
  way in both Auth and AuthHelpers.  This also differenciates it
  from the event "loggedIn".