Unhide methods in astroquery.gaia

[bsipocz]

E.g. currently we have __query_object that is called in both query_object and query_object_async, but kept inaccessible to look at from a python prompt. I see no reason for having all these three methods defined while the latter two only calls on the implementation in the first.

Instead, we should rename the hidden to query_object_async, and use the decorator to create the query_object, just as we do in the other modules.