Ruby 1.9.x support

I've made a couple of changes that would make the gem compatible with Ruby 1.9.x
* The Net::ReadTimeout and Net::OpenTimeout constants are not defined in 1.9.x and so there needs to be a const_defined? check
* There is no support for named parameters yet, and assigning nil as the value of a hash key is an error in 1.9.x

There shouldn't be any regressions with these two changes. However I don't know of any spec library that would allow me to test against different ruby versions (I just did manual tests).
I don't think work. The ruby 1.9 equivalent former would be something like:

```ruby
def initialize(options = {})
  original_exception = options[:original_exception]
  response = options[:response]
  #....
end
```