Return full gapi object

The JS library includes much more than just `gapi.hangout` so returning the full `gapi` reference makes more sense in my opinion.

I checked and `google-hangout-button` seems to be the only component that depends on `google-plusone-api` so far, I opened another PR there to go along with this change. (https://github.com/GoogleWebComponents/google-hangout-button/pull/5)