gemspec: update killbill-client-ruby to 0.2.0

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
sessions: integrate with Kill Bill sessions

After log-in, subsequent calls to Kill Bill are performed using
the session_id retrieved by the killbill-client library.

This also fixes various bugs in the Devise integration.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
models: add entitlement model

This is to support entitlement:* permissions.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
models: remove deprecated Kaui::ErrorHelper from account_email.rb

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>