Add support for configuring an addon when adding it.

Added support for configuring an addon when you are adding it to an application. This feature is present in the ruby wrapper, and is very useful. Tested using the deployhooks addon, as programatically specifying a hook url is useful.


Example usage:
```
cloud = heroku.from_pass("person@awesome.com","magic")
awesome_app = cloud.apps[-1]
awesome_app.addons.add("deployhooks:http",params{'url':'http://awesome.herokuapp.com/depHook'})

```