Do not use Session as it's Migrated across reloads

Just ran into this.

https://github.com/Differential/polymer-demo/blob/master/client/init.coffee#L3

https://github.com/meteor/meteor/blob/devel/packages/session/session.js#L1
https://github.com/meteor/meteor/blob/devel/packages/reactive-dict/migration.js

> ReactiveVars are not automatically migrated across hot code pushes, whereas Session state is.

http://docs.meteor.com/#/full/reactivevar
