For region queries, moved parameter parsing from _args_to_payload to query_region_async

There is now a center dictionary passed between the two.
Made query_object_async use center as well.
In _args_to_payload, caller is not needed anymore.
Merged height and width keywords of query_region_async into a single box keyword.
Made radius keyword of query_region_async support annular region.
Removed keywords from query_constraints_async

The functionality is already present through **kwargs.
In addition, in Vizier language, keyword means catalog keyword, not
column filter.
_parse_dimension now only works with astropy.coordinates.Angle

This also preserves the unit requested in query_region when requesting
the '_r' calculated column.
Simplified columns and column_filters in VizierClass constructor and _args_to_payload.

Not related to this commit, but at this point query_contraints does not
work properly.
Made query_constraints use _args_to_payload.
For some catalogs, if you do not specify the full sky (-c.rd=180), the query fails.
Fixed failing tests for columns and column_filters in Vizier.

columns and column_filters are not properties anymore.
Implement catalog in Vizier base class.
Fix: catalog, columns, and column_filters from Vizier base class were not used properly.
Implement Vizier sort functionality.

Now you can add a '+' or '-' to sort on a requested column.
disable stable tests on master (at least until 0.3 becomes "stable")
Merge pull request #246 from jwoillez/vizier_improvements

Vizier improvements
Merge branch 'master' of github.com:astropy/astroquery
change default NIST example
Add an example of input wavelengths.

Add an example of how the input wavelengths should be passed to the NIST
query. Related to #251.
Fix example.

The example query was from 40000 Angstrons to 70000 Angstrons. I have
changed that and I have changed the output query to reflect the same as
mine.