Fixed inconsistency between Mash#respond_to? and #method_missing. Added Hashie::Extensions::Mash::ActiveModel for compatibility with Rails 4 Strong Parameters.

I would appreciate comments on the README and the implementation. Maybe there's a way to do this automatically via Railtie or something like that (I don't know how)?

```ruby
class Mash
  include Hashie::Extensions::Mash::ActiveModel
end
```

Closes #146 and #89.