New "offline-accounts" package munges `Meteor.users` to be an offline
collection, and hacks Meteor.loginWithToken to not show the "logging
in" animation on startup.

Add an `Offline.wrapCollection` method to wrap a `Meteor.Collection`
and return an offline collection.

Updates "parties" example to use offline collections.  Change the
"createParty" method to use a party id generated on the client so
rsvp'ing a new party works while offline.