useful when model factories use factory.SubFactory() for foreign
key relations.

Enables checking of migrations and reports when there's an
AddField with a default value which may slow down applying
migrations on large tables. This may also lead to production tables
being locked while migrations are being applied.

It turns out the existing transformations were not registered.

Also add a test for #60.

The test can't reproduce the original problem reported in the issue
so consider it resolved.

the way we do this is by modifying our transformations to return
an empty list instead of None.

ModuleNotFoundError is a subclass of ImportError, introduced in
Python 3.6, and as such fails in lower Python versions.

https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError

we're down to mostly fix me and TODO items

this is for versions which are several years old and pylint-django
requires a relatively new pylint, which probably pulls in latest
astroid.

I'm not adding a versioned dependency for astroid. If anyone
reports a bug for that we can add the dependency and tell them to
upgrade!

in this case we expect no errors for setup.py

by ignoring the import failure Python will not produce a traceback
but instead let DjangoInstalledChecker do its job and warn the user.

I also add a CI build stage to test this scenario since we don't want
to continue if this isn't working. NOTE: tox doesn't allow us to easily
execute shell scripts and commands outside its virtualenv so I have to
resort to ugly bash escaping to make this work!

Finally properly fixes #96.

This commit adds an optional dependency for Django for users who
would like to have Django installed automatically when installing
pylint-django.

It also updates the text in README and adds an ISSUE_TEMPLATE which
warns the users not to report bugs if Django is not installed
automatically!

This reverts commit 9b520e9b.

Apparently this is causing more problems than it solves, see:
https://github.com/PyCQA/pylint-django/pull/132

also add a big warning statement about possible side effects and
to deter people from reporting issues about Django version
mismatches. Updates README formatting a bit as well.

- setup.py: move all the values directly as parameters to setup()
  to make it more readable and avoid scrolling back and forth to
  figure out what comes from where
- MANIFEST.in: add Markdown files to the list of included files,
  e.g. CHANGELOG, CONTRIBUTORS, README; adjust how we include the
  supplementary files for tests