appcache package

This code depends on PR 680.  In addition, the docs include a link to
the proposed AppCache wiki page.

Adds the appcache smart package and associated documentation.

QA notes are in packages/appcache/QA.md (Is this a good place to put
them?)
Avoid calling readFileSync at runtime.
appcache code review changes
add missing semicolon
Depends on PR 685 (the appcache package).

Enable the app cache for docs.

Move the "forkme" image to public/ so that it is available offline.
Allow static resources to be configured as online only.

Add a route policy type "static-online" for files in public/ that
shouldn't be cached offline.

Put "static-online" files in the manifest NETWORK section instead of
in the CACHE section.
Fix syntax error in appcache-server.

Ouch.
Also add static resources from packages to the manifest.

Static resources (such as images) added by packages weren't getting
added to the manifest.  This meant that e.g. the bootstrap glyph icons
weren't available offline.
(Fixes https://github.com/awwx/meteor-appcache/issues/1)