Fix SlimScrolling handler for AJAX scenarios.

If fullpage is invocated way after the window is loaded (e. g. when a page that is fullPaged is dynamically added to DOM) createSlimScrollingHandler() is never run.
To fix this I propose checking for document.readyState, not whether fullPage.js is already loaded.
Remove the 'plugin already used' mark

It's not usedin the code anymore
Merge pull request #722 from kojoru/patch-1

Fix SlimScrolling handler for AJAX scenarios.