Eliminate reliance on global env object

This is branched off of #19 and will resolve #14.

Changes in lib/auth are what we're interested in. Changes in addon/adapters/adapter.js are inherited from #19.

I think this is the least painful way to eliminate relying on a global ENV object - the GOOGLE_CLIENT_ID is simply passed in as a parameter of the authorize/authorizeImmediate options object.

Since evenutally, we also want to move the ember-simple-auth authenticator inside the addon, this should also make things simpler in the future.