add implementation of respond_to? for ActiveSupport::Duration

In rails 4.2.beta, calling respond_to? on a ActiveSupport::Duration no longer returns true for the methods defined in ActiveSupport::Duration. ie 

```ruby
10.seconds.respond_to?(:since) #=> false 
```

This is because the `since`, `ago` and similar methods were removed from Numeric.

This adds a custom respond_to? method to currently return if the duration responds to these methods.
@lsylvester See the code snippet above :grin: You have to `#unbind` the `Method` first, then `#bind` the `UnboundMethod` to the class.

Agree that `Duration` should simply be an `Object`. It's not really a transparent proxy anymore.