Fix up session initializer documentation and README

The example session code doesn't work, it errors with the following when you try to log in from the app's root:

ActiveRecord::RecordNotFound (Couldn't find Shop without an ID):
app/models/shop.rb:10:in `retrieve'

I fixed it up to return nil if id is blank.

Additionally, the example code in the README didn't have the self. update from the last doc update.

Russell.
Wouldn't `return if id.blank?` be sufficient?