pwkit/ndshow_gtk2.py: import and bugfix
pwkit/lsqmdl.py: import
pwkit/lmmin.py: import; and fix up lsqmdl
pwkit/radio_cal_models.py: import
pwkit/cli: turn into its own subpackage

Also correct __all__ variables, which should be bytes and not Unicode for
certain corner cases that I've been running into.
pwkit/cli/astrotool.py: import 'astrotool' command

I have to massage it a bit to work it into the setuptools system, but things
are pretty straightforward.
More helpful default usage for astrotool and radio_cal_models.