changed manifest extension to "appcache"

The file extension for manifest files should be changed to "appcache".  I have changed all instances of `.manifest` to `.appcache` and added a paragraph describing the change to the Offline page.  I'm not sure of the build process for the final site, so feel free to make whatever adjustments need to be made.  If code examples are packaged as zips, those should be updated as well.

I tried looking for a standard on this, 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