Renamed SitemapNamespaceConstants class
Removed xmlns:i
Formatting
Added gittattributes

LF normalization
Formatting
Formatting
Formatting
Added constructor for SitemapIndexNode
Added constructor for ImageDefinition
Renamed ImageDefinition
Sitemap node can contain many images

https://support.google.com/webmasters/answer/178636
Updated test with more than one image
Made SitemapModel ctor internal

Return empty list when sitemapNodes parameter is null
Removed redundant method
Using IXmlNamespaceProvider for namespaces

Classes used for serialization specify which namespaces are needed by
implementing IXmlNamespaceProvider interface
Addedi missing SitemapImage properties

https://support.google.com/webmasters/answer/178636
Fixed route configuration
Code formatting
Removed ImageDefinition property example
Added note for creating index files