add support to media attribute according to https://developers.google.com/webmasters/smartphone-sites/details
* Fix specs for removal of Sitemap Writer service
add expires tag
* Add specs for the expires tag
Changed S3 write implementation to a single call
* Don't include Fog as part of SitemapGenerator core.
* Run the S3Adapter test as an integration test, and exclude it from the regular test runs.
Include media attribute; Fix merge conflicts
Show more informative errors if Fog or CarrierWave have not been required when the adapter is loaded.
Add changes to README; Fix links
Upgrade to 4.3.0