Improvements to ActiveSupport::Notifications.

See [gist](https://gist.github.com/tgxworld/c1641dd18bea614ca216) for Benchmark.

Before:
![screenshot from 2014-06-01 14 13 57](https://cloud.githubusercontent.com/assets/4335742/3142738/b74d348a-e9d1-11e3-9e7c-965318186eae.png)

After:
![screenshot from 2014-06-01 14 14 07](https://cloud.githubusercontent.com/assets/4335742/3142740/bdddf6f4-e9d1-11e3-9d62-c103683bc326.png)