Add Simbad.query_objectids(), a method to return a list of identifiers

from Simbad as an astropy.table.Table.

In the process, split SimbadResult into per-output-type subclasses,
SimbadVOTableResult, SimbadBibcodeResult, and SimbadObjectIDs result.
The aim is to encapsulate result output parsing functionality into
specialized subclasses, and avoid messy if-else statements.
That's right, I'll update the documentation.