Replace custom lifecycle with AS::Notifications

By using ActiveSupport::Notificaitons, we can take advantage other tools already available that work with its interface. This will also make it easier for persons already familiar with ActiveSupport::Notifications to add logging, statistics, etc. for protobuf. After this change, I'd like to continue by adding additional instrumentation hooks throughout the code, so I can easily integrate it with logstash and statsd.

I know the increased dependency on activesupport is not ideal, but I think the benefits of having a uniform instrumentation interface is worth it. Rails (starting in 3.0 I believe) started using AS::Notifications for all of its logging, so I feel confident suggesting it is production hardened code.

Thoughts?

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