extract Throttle logic

Now we are able to given a special Throttle implementation, or use the Time one.

As explained on https://github.com/soundcloud/lhm/issues/23#issuecomment-20717297 we will be able to do this:

```ruby
# common implementation
Lhm.setup_throttle(:time_throttle, connection: delay: 5 )
# custom.
Lhm.setup_throttle(Shopify::LhmThrottle, options )
Lhm.setup_throttle(SoundCloud::LhmThrottle, options )
if Rails.env.development?
  Lhm.setup_throttle(Lhm::Throttle::Null, options )
end
```