Some slight tweaks to CompositeModels for the v0.3.2 release.  This doesn't add much improvement or fix any issues--instead it addresses issues like #1836 by being clearer about what these classes don't currently support.  It also slightly improves their text representations.
Remove some deprecated things in `units` and `wcs`.
Added link to fperez's git notebook
Merge pull request #2430 from bsipocz/fixing_failing_automodapi_test

Fixing failing tests in test_automodapi
Make Row, Column, MaskedColumn, TableColumns be class attributes
Add initial tests of subclassing components
Make the user-subclassable attributes be public (no _)
Refactor pprint funcs into class to allow formatting customization
Make subclass testing more boring
Add subclassed Table into test fixture for broad testing
Add docs on subclassing
Update CHANGES.rst
Add top-level reference to subclassing, fix doc problems
Change t.pprint() to print(t) to fix doctest errs on Mac
Fix RST docs to run correctly and be more clear
Change time format from iso to isot in time docs example
Removing leftover HEAD marks
Merge pull request #2435 from bsipocz/changing_time_example

Change time format from iso to isot in time docs example
Merge pull request #2386 from mdboom/misc/remove-deprecated-0.4

Remove things that were deprecated in 0.3
Merge pull request #2436 from bsipocz/fixing_stray_HEAD_marks

Removing leftover HEAD marks