Support private hipchat server

HipChat now supports privately hosted servers.

This adds a server field to the service.
Do you know a slick way to pass the option where it wont override the default?

If I just pass `server` and allow it to be nil it will I believe still override the default because of: https://github.com/hipchat/hipchat-rb/blob/master/lib/hipchat/client.rb#L13

```
{ a: 5 }.merge({ a: nil })
=> {:a=>nil}
```