Show helpful exception message when a HIERARCH keyword is too long
Update docstring of FITSImage.update_keyword()
FITSeeingImage.__init__(): catch ValueError raised by update_keyword()
Set keywords.sex_catalog to 'SEX-CAT' (non-HIERARCH keyword)
Set keywords.sex_md5sum to 'SEX-MD5' (non-HIERARCH keyword)
Use non-HIERARCH keywords in FITSeeingImage

PyFITS does not support CONTINUE for HIERARCH, which means that if our
keyword has more than eight characters, or contains spaces, its length
plus that of the equal sign and the value cannot exceed 80 characters.
This causes FITSeeingImage.__init__() to raise ValueError if the path
to the on-disk SExtractor catalog is longer than this limit. Avoid it
by setting 'SEX-CAT' and 'SEX-MD5' as the (non-HIERARCH) keywords used
by the FITSeeingImage class. Thanks to Jean-Baptiste Marquette for
letting us know about this problem (#11).

This merges branch 'HIERARCH-and-CONTINUE-exception-message'
Add print_exception_traceback() decorator
Decorate functions called through multiprocessing
Remove old debug comment
Show the stack trace of exceptions raised by map_async()

Exceptions raised in processes spawned by multiprocessing do not print
stack traces. This leads to indecipherable error messages, as the user
can only see that something went wrong in the main process, without a
clue of what and where it exactly happened. Use a decorator to capture
these exceptions and print their information to standard output before
raising them again.

This merges branch 'multiprocessing-tracebacks'