Add support for offset coordinates

I am reopening astrofrog/wcsaxes#8
I tried the `SkyOffsetFrame` solution, but the result is not satisfactory, considering this example :

```
import numpy as np
import matplotlib.pyplot as plt
from astropy.wcs import WCS
from astropy.coordinates import SkyCoord

header = {'CRVAL1': 15., 'CRVAL2': 20.,
'CUNIT1': 'deg', 'CUNIT2': 'deg',
'CRPIX1': 16, 'CRPIX2': 16,
'CTYPE1': 'RA---TAN', 'CTYPE2': 'DEC--TAN' }
wcs = WCS(header)
coord = SkyCoord(header['CRVAL1'], header['CRVAL2'], unit="deg")

ax = plt.subplot(projection=wcs)
ax.imshow(np.zeros((32, 32)))
overlay = ax.get_coords_overlay(coord.skyoffset_frame())
```

the overlay axes looks good for the latitude but is an angle module 360° for the longitude.


Start by reproducing the supplied WCS, SkyCoord, and get_coords_overlay example and inspect how the overlay handles SkyOffsetFrame longitude. Compare the longitude and latitude axis behavior, then define completion as offset coordinates displaying correctly without the longitude wrapping through 360 degrees.

Written by the indexing model from the issue text.