Check for and prevent initialization of NDData with Quantity

Closes #2374, though admittedly in an unsatisfactory way.
Add changelog entry
Fix docstrings in modeling. Add a note in wcs docstrings about origin of outputs coordinates
Make coeff keyword arguments so that a polynomial with default parameters zeroes can be initialized.
fix typo
Allow NDData and uncertainty to be set with Quantity
Tweak to remove unnecessary ' in coordinates index
Added missing "the"
Include astropy-helpers in the tarball.
Re-wrote frame documentation
Tweaks to consistent-ify the sgr-example with some recent changes
Update changelog, move entry to New Features
Add info about push.default=simple not available in git <1.7.11
Issue INFO-level warning if unit property is changed directly from a non-trivial value

Closes #2411
Changelog entry
add utilities for swapping or removing axes from a WCS
preserve metadata

don't need that velosys setting any more
added another utility for adding a stokes axis.  It's ridiculous, but
apparently it's needed for compliance with CASA's spectral axis
conventions

docstring fixes and import fix
__init__ and changelog updates
added two new utilities and some more docstrings.  The new utils come
from @ChrisBeaumont and @astrofrog

docstrings

more docstrings