Speed up fk4_no_e_fk4 test.
Mention acceptable shortcut for `list(six.iterkeys(d))`
Fix Python 3 issue on big endian platforms
Add CHANGES entry
Merge pull request #1891 from mdboom/docs/six-guidelines

Additional note to six coding guidelines
Merge pull request #1887 from mdboom/testing/python3-endianness

Fix for endianness test on Python
Merge pull request #1883 from mdboom/coordinates/speed-up-fk4-fk4-test

Vectorize test_fk4_no_e_fk4
Fixes issue 1912
Added bugfix for issue 1912 to CHANGES.rst for version 0.3.1