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 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
Use descriptor for ``meta`` attribute on Table, NDData, Column, and MaskedColumn
Removed unused code
Rename variable has_read to has_readwrite for clarity