Update for Meteor 0.6.5

Use api.export for exports.  (The `awwx` global variable for internal
exports is dropped entirely).

Use new versions of browser-msg and canonical-stringify which support
Meteor 0.6.5.

Add `Offline.isWebWorker` to indicate whether we're running in the
shared web worker.

Inside the shared web worker environment, use `WebWorker` instead of
`Agent`.