rack: fix logging bug

Under JRuby, System.out and System.err were randomly closed in finalize
calls, making all Kill Bill logs unavailable.

This patch delegates the rack logger and Error stream to our own logger,
which was a TODO anyways.

Signed-off-by: Pierre-Alexandre Meyer <pierre@ning.com>
Merge branch 'master' of github.com:killbill/killbill-plugin-framework-ruby