Adding autogenerated manifest and admin service

1. Adds hooks for an API with the following calls:
 * manifest: Returns the manifest file
 * loader: Returns the appcache-loader.html for embedding in an iframe.
 * update: Forces an update of the manifest file

2. Automatically generates the manifest file when a file in the WWW directory changes.

3. Provides a pocket admin panel for the plugin, which shows the manifest file and an option to update the manifest.

Fixes gr2m/hoodie-plugin-appcache#1 & gr2m/hoodie-plugin-appcache#2

Creates these issues which I will add once merged:
 * Change appcache-nanny to support a custom iframe source, then use the plugin API as the source.
 * Better detection of the WWW folder for tracking changes. Needs hoodie modification.
 * Improve directory management for autogenerated files, e.g. appcache-loader.html, manifest.appcache
No, because the path to the application cache has to be different than the default appcache nanny provides in the file. Instead, I'm going to put the text for the appcache loader into config for the plugin and serve it from there.