Make Repository comparisons work

Firstly, the __lt__ method was broken.
Secondly, I hadn't specified total_ordering for the class.
Thirdly, I hadn't provided a __ne__ method.

Fixed.
Make muddle complain "nicely" if not Python 2.7

We no longer work with 2.6, and we definitely don't work with 3.x, so
try to say so in an obvious manner.
Correct quoting in test failure message
Sorting of repositories changed - fix test
Fix "empty" print statements

'print("")' is the Python3 compatible equivalent of 'print' - in Python2
'print()' prints out '()'

(this is in the context of making __main__.py be legal Python3 code so
that it can report it doesn't support Python3)
Making much work out of little
Make "muddle status" etc cope better with nosuchdir

If the directory for a checkout doesn't exist, then the VCS
commands (including "muddle status") should report it rather
better.
Muddle status should say if checkout not checked out

and in general keep going for more things
Very basic weld support.
Add Choice and other stuff

The new Choice class is intended to allow choosing one of several
options depending on a target string.

In support, add a mechanism for extracting information from
/etc/os-release (which Python's platform.py doesn't yet look at),
and thus get_os_version_name(), to do more-or-less what it says.
Add the ability to aptget based on OS version

This uses the new Choices mechanism to allow choosing a
package on the basis of the available OS.

It's possible to specify the OS version name just in case we
want to do this for cross compilation (of an Ubuntu system)
Some basic testing of the Choices and aptget changes

Note that for aptget in particular it really only checks that
build descriptions are valid or not - it doesn't actually check
that the aptget call produces the correct internal result.
"muddle doc <class<" should report on __init__

Before, there was a bug in this, and it just repeated the text
for the class. Now it reports the text for the __init__ method
after the __init__ signature.
Refine the docstrings a little
And fix a typo or two
Merge branch 'master' into issue247-improved-cmd-running

Conflicts:
	muddled/__main__.py
	muddled/pkgs/aptget.py
and also tests/test_choices.py

...all fixed

Tests all pass
Remove the (deprecated) linux_kernel.py

This was marked deprecated some while back. It has not been
maintained, and may or may not work. If you need it, use an
earlier version of muddle.
Sort out what I'm doing with "utils.shell"

(at least for now)

Comparison of the test outputs from old muddle and this new version
suggest this may be a sensible version to go with.