Minor cleanup of `astropy.vo` docs
Fixed ExpatError caused by mal-formatted USVO query XML as mentioned in astropy/astropy#1664. Removed a stale Cone Search service that has been giving Connection Refused for months.
Add registry.get_formats() function to return table of formats
Check for no formats matching input data_class
Auto-update unified I/O class read and write docstrings to show available formats
Include available formats in exception message for read/write fail

Some minor comment additions and docstring update as well.
Fix minor doc mistake
Add deprecation of short-form ascii format names
Add formats table to unified I/O docs
Initial implementation of descriptions for parameters, which also override their doc strings--in the long term this will be nice for both interactive help and generated API docs.
Add instance name to built-in cosmo docstring, output class name with __repr__

* For the built-in cosmologies (like WMAP9), add information to
  the docstring stating they are instances of built in classes,
  and identifying the class.
* Change __repr__ to print the type of class first, then the name
  as an argument (e.g., instead of WMAP9(...), FlatLambdaCDM(name="WMAP9", ...)
* closes 1544
Add test for OrderedDict subclass as NDData.meta
Fix the meta tests
Make Table.meta and NDData.meta behave the same
Use same tests for Table and NDData meta
Make doc update work without pre-existing formats table
Fixes so that test_registry tests pass
Fix Python 3 failure when updating docstring
Update to output (-h,-m,-s) in .hms and .dms, for consistency w/ 0.2
update tests for corrected Angle.hms/dms behavior