change file extension to "appcache"

[CoffeeAndCode]

The file extension for manifest files should be "appcache".

I tried looking for a standard, but only found the following resources supporting it:

• http://appcachefacts.info/
• http://stackoverflow.com/questions/6523034/html5-appcache-manifest-file-extension
• http://html5.org/tools/web-apps-tracker?from=5811&to=5812
• http://en.wikipedia.org/wiki/Cache_manifest_in_HTML5
• http://www.w3schools.com/html/html5_app_cache.asp

and the following is indifferent:

• https://developer.mozilla.org/en-US/docs/HTML/Using_the_application_cache

[CoffeeAndCode]

Closing as a new issue (#49) was created with the pull request.