Use nose test runner and fix broken test

I fixed the following issues with the tests:
- They did not run on python 2.6 => I changed the test runner to `nose`. This fixes #101.
- They were very slow because they fired many http request, in fact `test_parse_preview` was disable for this reason. => The grab_video_url and get_video functions are now replaced with a mock function.

I also removed the `__init__.py` file from the top level directory and moved the tests the coursera package.

More details are in the commit messages.