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.
@jeremy I think implementation of `respond_to?` returns true based on if the class has the methods defined instead of the the instance.

I don't think that it is possible to bind a method to a `ProxyObject` from `Object` as I get an error when I try to do so.

```ruby
Object.instance_method(:respond_to?).bind(BasicObject.new)
 #=> bind argument must be an instance of Object (TypeError)
```
Maybe `Duration` should be a subclass of `Object` so it can get the correct `respond_to?` behaviour for free. 

The only issue identified by the tests when `Duration`s superclass is changed to `Object` is that it can now be duplicated, which can be fixed by defining `duplicable?` and undefining `dup`.

Do you know any reasons why `Duration`s superclass shouldn't be changed to `Object`.