Do not use the 'with' statement with pyfits.open()

Although we require version 2.0 (released January 2009) or later of
PyFITS, support for the with statement was not added until version 2.3
(May 2010). This means that users with versions older than that get an
AttributeError exception (HDUList' object has no attribute '__exit__')
if the with statement is used when opening a FITS file.
Add sextractor_version()

Up until now we have blithely run SExtractor without paying attention
to which version is installed on the system. In case we need to know
the version number, this function parses the output of the --version
option and returns it as a three-element (major, minor, micro) tuple.
Add unit test for sextractor_version()
Require SExtractor version 2.8.6 or newer

The syntax that, when working with multi-extension FITS files, makes
it possible to select on which one sources are detected was not added
until SExtractor 2.8.6. Earlier versions interpret the square brackets
surrounding the extension index as part of the name of the FITS file,
which results in error messages such as "cannot open IC5070.fits[0]".
In order words: since commit b1669cd8b0f19 sextractor() does not work
on systems with versions of SExtractor older than 2.8.6.

Add the module-level variable SEXTRACTOR_REQUIRED_VERSION, set it to
the three-element tuple (2, 8, 6) and make sextractor() raise a new
exception, SExtractorUpgradeRequired, if the version installed on the
system is older than this one.
Test that SExtractorUpgradeRequired is raised by sextractor()
Include the minimum required version of SExtractor (>= 2.8.6)

The version of SExtractor provided in the stable repository of Debian
Squeeze is 2.4.4, but we now require 2.8.6 or newer. This means that,
at least until the next release of Debian (7.0), SExtractor cannot be
installed using apt-get. We need to download the binary package from
the Astromatic website.
Add running the test suite as the last step of the installation
Fix typographical error