Add 'session' to auth hash in OAuth complete method

If you're creating an authenticator as a plugin, it may be useful to have access to session data at some point during the authentication process.  This could be during either the after_authenticate or after_create_account methods.  Per Sam's suggestions, I am including session data within the auth hash found in the OAuth complete method.

In our case, the way our current_user method works is by checking if a certain token exists within a session before each page load (we do not store the token in the database as our security team does not want us to do so).  A few members of the Discourse team have access to our repo to take a look at how we'd use this: https://github.com/newrelic/forum/blob/new_user/plugins/golden_gate_client/plugin.rb