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
I wonder if we need to write the manifest.appcache file at all? Maybe we could store all files from last `www/` scan in the `hoodie-plugin-appcache` database, together with hashes based on their contents? Then only maintain the version in memory and serve it dericetly at the `/manifest.appcache` hook URL?

We can also merge this for now and discuss this in a separate topic