Added to the scope JS 101 article

Following on from https://github.com/jquery/learn.jquery.com/issues/89#issuecomment-10007986, I finally found some time to quickly move over some of the content I wrote for my initial [Scope and this in JS](http://javascriptplayground.com/blog/2012/04/javascript-variable-scope-this) article and move it over into the Scopes article, reworking it slightly but not making any major changes.

Let me know what you think :)