sextractor_md5sum(): turn 'options' parameter into a dictionary

A dictionary is a much more robust data structure than a list here, as
it guarantees that each SExtractor parameter (key) has a corresponding
value. This is less brittle than using a list, where two elements are
needed for each parameter-value pair, and prevents the ugly scenario
in which the user may forget to add to the list either the name of the
parameter or its value, resulting in sometimes-obscure error messages
(for example, if the parameter is missing, SExtractor will assume that
the value is a second FITS image and try to run in double-image mode).
Improve docstring of sextractor_md5sum()
Add unit test for sextractor_md5sum()
Add python-mock (Debian package) to the installation instructions
Remove unused import
Fix references to Debian — it would be Debian GNU/Linux, if anything
Raise an exception in sextractor() if a configuration file is missing

The latest stable version of SExtractor, 2.8.6, aborts the execution
if either the PARAMETERS_NAME or FILTER_NAME parameters contain the
path to a file that cannot be read. But, in the case of STARNNW_NAME
and the configuration file (-c option), it prints a warning and uses
the internal defaults. In order to treat the four configuration files
equally and make the function more robust, check that they exist and
are readable before calling SExtractor, raising the IOError exception
if any of them is not.
sextractor(): turn 'options' parameter into a dictionary

The reasons for this change are the same as those explained in commit
8d5bf4fed4a7cef4. The name of the parameters do no longer need to have
a dash prefixed — what until now was ['-DETECT_MINAREA', '25'] becomes
{'DETECT_MINAREA' : '25'}, for example.
Improve docstring of sextractor()
Download sample FITS images from the STScI Digitized Sky Survey

In order to keep the size of the Git repository as small as possible,
the sample FITS images used by some unit tests will be automatically
downloaded from the Digitized Sky Survey. This process is transparent
to the user, which only needs to know what the module-level variable
TEST_IMAGES contains the path to some FITS images.
Show percentage complete when a FITS image is downloaded
Download sample FITS images before running the unit tests
Add unit test for sextractor()