Minor improvements to usage message reformatting

Remove undocumented .strip() from join_lines()

Why I noticed this: It has the potential to eat significant white
space in usage messages that are marked up in reStructuredText syntax.

Why I decided to change it: The behavior isn't documented and on
second thought I wouldn't expect a function called join_lines()
to strip any and all leading/trailing white space.

Improve usage message parsing algorithm (also add a proper test)

Refer to test_parse_usage_tricky() for an example of a usage message that
is now parsed correctly but would previously confuse the dumb "parsing"
algorithm in parse_usage().

Make usage message parsing a bit more strict

Admittedly this needs a lot more love to make it more robust but
I lack the time to implement this at the moment. Some day soon! :-)

Unbreak conditional importlib dependency*

* https://travis-ci.org/xolox/python-humanfriendly/builds/110585766

  Kind of strange that everything worked fine locally. Then again,
  I've never used wheel compatible conditional dependencies before
  so of course my first attempt was bound to fail in same way :-s.

Make conditional importlib dependency compatible with wheels*

While running tox tests of another project of mine that uses the
humanfriendly package I noticed a traceback when importing the
humanfriendly package (because importlib was missing). After some
digging I found that tox uses pip to install packages and pip converts
source distributions to wheel distributions before/during installation,
thereby dropping the conditional importlib dependency.

Fix non-fatal log format error in prompt_for_choice()

Add humanfriendly.sphinx module

Bug fix: Restore Python 2.6 compatibility (doh!)

Replace import_module() with conditional dependency on importlib

[less code = better code base :-]