This commit removes the dep introduced in 5e12233 by just importing some tiny
bit of dirspec code.

The previous change was introduced because:

  * pyxdg did not account for Mac OS specifics, i.e. using ~/Library/
    directory structure instead of .config (see:
    https://leap.se/code/issues/3574).

  * dirspec does the correct thing for xdg on Mac OS.

  * u1db depends on dirspec anyway.

The problem is that dirspec is not maintained and published on pypi, what
forces us to download it from an URL and add exceptions to be able to pip
install it.

As we are removing dependence on u1db on other modules, we can also remove it
here. To workaround the Mac OS problem, we just add some code from dirspec to
ensure we get the correct directory on Mac OS.

Also added dirspec directly to the setuptools for now,
because it needs the dependency along with the url, but pip
would break if it had both

Leap pycommon will now be installed in editable mode
when you use pip install, so that you can run the tests
and develop using only pip

I also added a conditional to the setup.py so
that python setup.py develop can be used even
with the dirspec url

--use-leap-wheels sets --trusted-host (remove it when we have a proper
  cert) and WHEELHOUSE to https://ftp.lizard.leap.se

Until we get ftp.lizard cname, use lizard as the wheels server.

generate_wheels uses $WHEELHOUSE to generate and store the wheels for
requirements.pip and requirements-testing.pip (if it exists).

pip_install_requirements.sh installs requirements.pip from them if
possible (if not, then it fetches them from pypi) or, if passed the
--testing flag, it installs requirements-testing.pip.