Update contentloaded.js

Set adder & remover automatically, based on feature detection.
- This ideally should be external to contentLoaded function, so it is not run on each fn addition.

Then, rely on adder & remover functions to add or remove the event handlers
- This allows you to then use external adder / remover functions if you want.

Apply required events to correct browsers:
- modern browsers do not need readystate to be added, as they can reliably use DOMContentLoaded
- ie 6 - 8 need to use readystatechange, as they don't have DOMContentLoaded
- all browsers have onload event handler added as a fallback

next version will add a closure & IIFE to allow multiple functions to be added