Fix weblate.trans.tests.test_models:ProjectTest failure on subsequent runs.

Check_rename will fail on subsequent runs from same working directory due
to old_path (which is default project path used by many tests) not being removed from previous run:

 Traceback (most recent call last):
  File "/home/vrusinov/workspace/weblate/weblate/trans/tests/test_models.py", line 209, in test_rename
    self.assertFalse(os.path.exists(old_path))
AssertionError: True is not false
Add some logging to PathMixin's check_rename().
Fix some PEP8 errors and typos.
Merge pull request #469 from vrusinov/fix_rename_test

Fix weblate.trans.tests.test_models:ProjectTest failure on subsequent runs.
Share one logger

Signed-off-by: Michal Čihař <michal@cihar.com>
Update stats on unit content change

Even when there was no change on checks, changed content could lead to
different state of checks than it was before in case there are more
units with same contentsum within the project.

Signed-off-by: Michal Čihař <michal@cihar.com>