Merge pull request #2110 from amras1/time-docstring-formats

Documentation for time formats
Merge pull request #2368 from embray/fits-merge

Merge branch '3.2-stable' into astropy
Conflicts:

	astropy/io/fits/card.py
	astropy/io/fits/header.py
	astropy/io/fits/tests/test_checksum.py