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.
Merge branch 'master' of https://github.com/astropy/astroquery

Conflicts:
	astroquery/simbad/core.py
Update call signature of Simbad._parse_result()
Clarify query_objectids() documentation
Add tests for query_objectids() and query_objectids_async()
Add example for query_objectids()
fix remote tests, add one more remote tests for the PR