Uploader, multiple file dialogs, resolves #1277

This fixes #1277.

Here's a reproduction of the issue: http://jsfiddle.net/vzDKZ/

So, the reason two file dialogs pop up is because of:

    uploader.set('selectFilesButton', Y.one('#selectFilesButtonContainer'));

The `click` event is fired once on the `input` element itself, and then propagates up to `#selectFilesButtonContainer` and fires again, causing two file dialogs to pop up. Simply stopping propagation would resolve this issue.

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As far as testing goes: it's really hard. When the `click` event fires, a file dialog is going to appear. This is a big no-no when performing automated testing.