killbill_helper: fix indentation

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
gemspec: cleanups and add dependency on killbill-client

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
error_helper: propagate errors from killbill-client

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
payment_methods: switch to killbill-client implementation for API calls

This fixes also various bugs in the different screens and actions.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
payment_method: make add payment method call work with killbill-client

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>