can not parse the fits header with more than 2 axis

It can not work with the fits file which has more than 2 axis, like in radio data.
For example, a header as below :

```
SIMPLE = T /Standard FITS
BITPIX = -32 /Floating point (32 bit)
NAXIS = 4
NAXIS1 = 1600
NAXIS2 = 1600
NAXIS3 = 1
NAXIS4 = 1
EXTEND = T
EQUINOX = 2.000000000000E+03
RADESYS = 'FK5 '
LONPOLE = 1.800000000000E+02
LATPOLE = -3.529166667528E+00
CTYPE1 = 'RA---SIN'
CRVAL1 = 4.201666666401E+01
CDELT1 = -8.333333333333E-05
CRPIX1 = 8.010000000000E+02
CUNIT1 = 'deg '
CTYPE2 = 'DEC--SIN'
CRVAL2 = -3.529166667528E+00
CDELT2 = 8.333333333333E-05
CRPIX2 = 8.010000000000E+02
CUNIT2 = 'deg '
CTYPE3 = 'STOKES '
CRVAL3 = 1.000000000000E+00
CDELT3 = 1.000000000000E+00
CRPIX3 = 1.000000000000E+00
CUNIT3 = ' '
CTYPE4 = 'FREQ '
CRVAL4 = 1.519499768816E+09
CDELT4 = 1.024001037953E+09
CRPIX4 = 1.000000000000E+00
CUNIT4 = 'Hz '
PV2_1 = 0.000000000000E+00
PV2_2 = 0.000000000000E+00
```

It will cause the error:

```
File "build/bdist.linux-x86_64/egg/pyregion/__init__.py", line 57, in as_imagecoord
File "build/bdist.linux-x86_64/egg/pyregion/ds9_region_parser.py", line 184, in sky_to_image
File "build/bdist.linux-x86_64/egg/pyregion/wcs_helper.py", line 231, in _get_radesys
ValueError: too many values to unpack

```