Add tests for endpoints with authentication.

To run those tests you have to define USERNAME, PASSWORD and REPO_SLUG in `bitbucket/tests/private/__init__.py` , then run with `site-packages$> python -m bitbucket.tests.private.run_test` .

Also correct some errors made during split of bitbucket class.

Signed-off-by: Baptiste Millou <baptiste@smoothie-creative.com>
Version bump to 0.4.1 .

Signed-off-by: Baptiste Millou <baptiste@smoothie-creative.com>