Factor out the creation of the OptionParser instances

The optparse.OptionParser instances used in all of the modules of the
pipeline are always created in the same way: using as description that
of the module and the style.NewlinesFormatter class for printing help
text. Avoid this redundancy with a new module, customparser.py, which
implements the get_parser() function: it takes as its only argument
the description and creates and returns the OptionParser instance.
Do not add a help option to the OptionParser returned by get_parser()

Make optparse not to add the help option (with option strings -h and
--help) to the OptionParser instance that customparser.get_parser()
returns. To print the help text it is enough to type the name of the
command without any arguments, such as `lemon offsets`.

Anyway, even if we wanted to use them, there is the problem that the
--help argument causes 'lemon' to display its own help text and exit,
so there is no way that this option string can be passed down to the
module whose main() function is executed.
Remove short option string of the --follow option from import.py

As per the Zen of Python, explicit is better than implicit, so other
than -v (for --verbose) we are not going to use short option strings
anymore in any of the modules. Since support for Bash completion is
about to be added, users should not even be allowed to complain that
after this they have to type a lot more.
Add Bash completion support for the 'import' command

This is the script that will eventually provide completion support to
our code. It must be copied to /etc/bash_completion.d/ in order to be
automatically sourced, but, for now and until setup.py takes care of
that, when LEMON is installed, it can also be sourced directly (for
example, from your .bashrc). This very first version only completes
the 'import' command and its options.
Remove short option strings from seeing.py — and expand long ones

The logic behind this commit is the same that in be5ae81215e52f345ca7:
short option strings are deleted (or, in the case of options with only
a short option string, replaced with a long one), except for -v. Also,
those long option strings whose meaning is not clear enough ('--sper',
for example) are expanded so that they become understandable at first
sight ('--sources-percentile', in this case).
Add Bash completion support for the 'seeing' command
Remove short option strings from offsets.py — and expand long ones

The reasons for this commit are the same that for d4572bf76e05a95d.
Add Bash completion support for the 'offsets' command
Remove short option strings from mosaic.py

The reasons for this commit are the same that for d4572bf76e05a95d.
Add Bash completion support for the 'mosaic' command

Note that, when the --check-type option is used, we also complete the
different types of check-image that SExtractor can generate. The list
has been taken from the SExtractor User's Manual (v.2.13), page 11.
Remove short option strings from astrometry.py

The reasons for this commit are the same that for d4572bf76e05a95d.
Add Bash completion support for the 'astrometry' command
Remove short option strings from photometry.py — and expand long ones

The reasons for this commit are the same that for d4572bf76e05a95d.
Add Bash completion support for the 'photometry' command
Remove short option strings from diffphot.py — and expand long ones

The reasons for this commit are the same that for d4572bf76e05a95d.
Add Bash completion support for the 'diffphot' command
Remove short option strings from periods.py — and expand long ones

The reasons for this commit are the same that for d4572bf76e05a95d.
Add Bash completion support for the 'periods' command
Remove short option strings from annuli.py — and expand long ones

The reasons for this commit are the same that for d4572bf76e05a95d.
Add Bash completion support for the 'annuli' command