I've always found it awkward to call document.getElementsByClassName within a certain page context. Now that we've got Element.extend, we don't really need the awkward syntax of specifying the context in the second argument.
This patch adds two new methods to Element.Methods. The first lets you do $('container').getElementsByClassName('item') instead of document.getElementsByClassName('item', $('container')).
The second lets you replicate the $$ function within the children of a particular element. Instead of $$('#container p.item'), you can do $('container').getElementsBySelector('p.item'), which is handy when you're grabbing an element programmatically (or one that can't be uniquely identified by a CSS selector).
Sorry for not including test cases -- I'll add them as soon as I get the time.
