option to deny requests

I need the possibility to allow/deny serving the appcache manifest based on the clients request. Therefore I added the possibility to set Meteor.AppCache.config.disableRequest to a function, which gets asked for every request if the manifest should be served or not.
Background: We have a public website which we also sell packaged as an android app. Only if you buy the app you shall get the possibility to use it offline. I accomplished this by sending a cookie with a magic hash code in it and detecting it in the disableRequest function.

I also added a description for the Meteor.AppCache.config.addPath functionality I added some days ago. Sorry for the combined pull request. I just realised that you can't select which commits to use in a pull request. If you only want the description and not the disableRequest functionality I resubmit only this part via a separate branch.