Support dzi xml with namespaces

The DZI I'm working with has namespaces in the xml, and openseadragon doesn't recognize it as valid DZI.  These two changes were all I needed to get openseadragon to recognize and work properly with my DZI file.  It's possible there are other places where it's not still namespace aware, but I think the other methods used (getElementsByTagName, getAttribute) aren't affected by the presence of the namespace prefixes on the elements, and as far as I could tell everything was working.

Here's a sample of my dzi xml with namespaces:

```xml
<dz:Image xmlns:dz="http://schemas.microsoft.com/deepzoom/2008" TileSize="256" Format="jpg" Overlap="1">
<dz:Size Width="1458" Height="2246"/>
</dz:Image>
```