Add sitemap namespace awareness.

[grokwich]

The code does not appear to support namespace use (sitemap, index, rss, atom).

Here is a pull request that addes namespace aware parsing.
#173

Thx.

Here is an example sitemap.xml used for a test that was failing, but now passes...

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<sit:urlset xmlns:image="http://www.google.com/schemas/sitemap-image/1.1" xmlns:video="http://www.google.com/schemas/sitemap-video/1.1" xmlns:sit="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.w3.org/1999/xhtml link.xsd http://www.google.com/schemas/sitemap-video/1.1 video.xsd http://www.sitemaps.org/schemas/sitemap/0.9 sitemap.xsd http://www.google.com/schemas/sitemap-image/1.1 image.xsd">
    <sit:url>
        <sit:loc>http://www.example.com/1</sit:loc>
        <sit:changefreq>daily</sit:changefreq>
    </sit:url>
    <sit:url>
        <sit:loc>http://www.example.com/2</sit:loc>
        <sit:changefreq>daily</sit:changefreq>
    </sit:url>
</sit:urlset>