Expanded VO catalog and database functionalities. Moved VO exceptions to a separate module. Added tests for new functionalities. Changed inheritance to super.
Updated astropy.vo.validator to use new VO database API. Separated validate and inspect tests. Added new exceptions module under validator.
Fixed codes for Python 3 compatibility and to complete coverage.
Re-organized astropy.vo docs so it is less confusing.
Added documentations for the new VO catalog and database functionalities and updated existing ones. Added VOSDatabase.list_catalogs_by_url() method and updated tests. Other code updates so examples work.
Use double ticks on change log as suggested by Erik Bray.