WCS: handle non-standard keywords by default

As suggested in #583:

This changes astropy.wcs so that when reading WCS FITS keywords, it supports handling non-standard keywords by default.  

When writing out FITS headers, the new default is WCSHDO_safe, defined by wcslib as "write all extensions that are considered to be safe and recommended".

This doesn't break any unit tests -- I wonder whether it will cause files to fail that used to work, though.  I think (as usual) our users will help to find wacky FITS files for us ;)