Make site a url argument

This addresses the apparent api bug which affected both the retrieval of
favorites and the site-switching. Se #82

As far as I can see, this issue affects ALL api calls, so it seems
reasonable that it should be fixed inside `sx-method-call`.

The usage of sx-method-call stays the same, only the internal has changed.

We should still ask about this on stackapps. And we may or may not want to wait for an answer there before merging this.