Feature Proposal: Raise on some HTTP status codes

I would like if HTTParty throw an exception on some HTTP status codes. I thought in an option that would turn this feature on and define what status codes would throw exceptions.

Something like this:

``` ruby
class Foo
  include HTTParty
  raises_on status_codes: (400..599).to_a
end
```

What do you think of this? Can I send a pull request following this idea?
