sitemap video price support
Fixes #118: append slashes to relative paths

Fixes #117: add video price support
* Use latest Nokogiri that supports Ruby 1.8.7
* Move schema URIs to a constant on the module.
* Add prefixes to the XML fragments being validated so they validate.
* Add latest Video schema
* Fix PageMap specs

Refs #116: Quote the ping URL so it's obvious what content is

Fixes #113: Support symbols as well as strings for most arguments to add()
Upgrade to v4.2.0