Fix url transform for preview view

Accessing preview videos using the `view?lecture_id=` format was giving errors.
Add test_clean_filename_minimal_change case for marketing-001

closes #210
Merge pull request #211 from jonasdt/test_marketing-001

Add test_clean_filename_minimal_change case for marketing-001
Merge pull request #202 from srijan/patch-1

Fix url transform for preview view.

Thanks Srijan Choudhary for the contribution (and keep them coming!)
gitignore: Ignore files generated by python-coverage.
requirements: Add python-coverage as requirement for code coverage.

We should get our code covered by as many tests/execution parts as possible
to ensure that we deliver a good experience to our users and to make sure
that we don't have any lingering bugs out there.
travis: Hook `coverage` with `nosetests`.

We limit the package that we are interested in to only `coursera` (maybe to
be changed to `coursera_dl` after a potential merge with other projects?).

Given that the code tested is *not* 100% the same as the code executed
when really downloading things (due to stubs etc.), we need to check what
options we have to cover real execution as well.

Note that this still is not sufficient to get the results in
https://coveralls.io/r/coursera-dl/coursera as we need to communicate the
results to them.
README: Make coverwalls happy with the badge included in the README.

Otherwise, I get a huge banner telling me that I should include this.
travis: First shot at connecting with coveralls.io.
coverage: Add configuration for python-coverage.
Add a Bitdeli badge to README
Merge pull request #214 from bitdeli-chef/master

Add a Bitdeli Badge to README
coverage: Fine-tune what is included in the tests.
coverage: Annotate code that our testbot won't be able to run.
README: Make clearer that usernames are emails. Closes #213.
README: Put bug reporting instructions in a separate section.
README: Clarify quoting/escaping password characters from shells.

This is inspired by the discussion in issue #213.
README: Mention cleaning the cache as part of troubleshooting.
README: Reword the introduction of the troubleshooting section.