Add support for SiteMaps to framework

The solution should consider the following:
- [ ] Generation of the SiteMap.xml file that provides an index for individual SiteMap files (see example below)
- [ ] Generation of multiple gzipped SiteMap files (see example below).   These should follow the naming convention sitemap.xml.gz, sitemap1.xml.gz, sitemap2.xml.gz, etc.
- [ ] Each sitemap.xml file should be limited to 50,000 rows
- [ ] The SiteMapGenerator component should be pluggable to allow for SiteMapGeneratorPlugins
- [ ] Include generators for Categories, Products, and Pages
- [ ] Document adding new generators
- [ ] Generators can be configured with priority and change frequency settings


#Sample Sitemap Index file
```xml
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
  <sitemap>
    <loc>http://www.mysite.com/sitemap1.xml.gz</loc>
    <lastmod>2012-10-10T04:03:12Z</lastmod>
  </sitemap>
  <sitemap>
    <loc>http://www.mysite.com/sitemap2.xml.gz</loc>
    <lastmod>2012-10-10T04:03:12Z</lastmod>
  </sitemap>
</sitemapindex>
```

#Sample sitemap file
```xml
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9        http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">

    <url>
      <loc>http://www.broadleafcommerce.com/index.html</loc>
      <lastmod>2009-11-07</lastmod>
      <changefreq>weekly</changefreq>
      <priority>0.5</priority>
    </url>
</urlset>