Implement ActiveSupport-based Instrumentation

Hello,

this PR implements ActiveSupport Notifications support. Everything is entirely optional - if no `active_support/log_subscriber` is available, the code won't run.

The instrumentation gives a Rails application query and timing information for YT queries. Example:

```
  [...]
  Rendered app/cells/structure/abstract.html.erb (0.1ms)
  YouTube: GET http://gdata.youtube.com/feeds/api/videos/cl_0af30JXw?v=2.1&key=[scrubbed]  (239.8ms)
  [...]
Completed 200 OK in 320ms (Views: 76.9ms | ActiveRecord: 1.4ms | YouTube: 239.8ms | ...)
```

Cheers,

-- @vjt