pylint-gui was removed, because it was deemed unfit for being included
in *pylint*. It had a couple of bugs and misfeatures, its usability was subpar
and since its development was neglected, we decided it is best to move on without it.

Part of #975

Use the isort library for wrong-import-order checker.

Enable coverage report for coveralls

There's no such thing as "GNU Public License";
GPL stands for "General Public License".

Also, removes README.python3, which doesn't provide any extra information
that's not already found in the README.

Improve the install section of the README, by specifying pip and how to use the master development head.

--HG--
branch : jfleach/removed-broken-link-and-replaced-with-th-1436980631932

[doc] move contributors in a separated file, reorganize it a bit and give more credits to current contributors

fix a few places where astng was still referenced instead of astroid + bad logilab-astroid in pkginfo dependencies

triggered whatever the format using a command line option

mention and document a bit epylint, pylint-gui and symilar
gui.py : fix line length
debian/control: Suggests instead of Recommends python-tk

* new W0212 message for access to protected member from client code
    * new W0105 message extracted from W0104 (statement seems to have no effect)
      with string

forget the past.