option to add content of folders (Meteor.AppCache.config.addPaths)

for each given addPaths: if it's a file, add it, if it's a directory, add the contained files
remove .# path prefixes to support adding folders hidden from meteor (see https://github.com/meteor/meteor/issues/437)
recursively walking into subdirectories not supported by now

files get added to Meteor.settings.appcache.addedFiles