Clarify that Function.bind example will not work in many browsers

[addyosmani]

Function.bind is mentioned in the following locations:

https://github.com/jquery/web-learn-jquery-com/blob/master/content/javascript-101/closures.md
https://github.com/jquery/web-learn-jquery-com/blob/master/content/javascript-101/this-keyword.md (actual example)

As per the ticket subject, we should probably expand on this to cover the fact it won't work in older browsers.

May integrate some of the MDC compatibility notes here as a part of this: https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/bind

[addyosmani]

A section on Function.bind has been added with the notes regarding browser compatibility. https://github.com/jquery/web-learn-jquery-com/blob/master/content/javascript-101/closures.md

Please feel free to edit or re-arrange if you feel it can be better presented :)