Add support to incorporate image specific tags to a sitemap for string rendered output.

Check out http://support.google.com/webmasters/bin/answer.py?hl=en&answer=178636 for refernce.
Add image support to all the renders.

- Update Specs
- Fix bug regarding builder render
Add proper namespace prefix for image to the MAP_SCHEMA_OPTIONS hash
Merge pull request #12 from thrillcall/master

Add support for image specific tags to a sitemap