allow generating async/sync only proxies

Currently the dbus_proxy generates both async/sync variants which is not often needed as the user might only want the async variant.

This issue is mostly to discuss whether you're open to such possibility and what would make the most sense as an API. Maybe as an attribute to dbus_proxy to either enable/disable the sync variant, that's enabled by default and always generate the async one (as it's API is much nicer to use, especially once #142 (closed) is fixed)

If the sync variant is disabled, we can just omit the Async prefix from the generated proxies. I'm open to provide a patch for this if that's the direction you want to go with.