Refactor test_parsing

Add assert_parse and remove global variables.
Fix downloading multiple resources with the same format

The resources of a lecture were stored in a dictionary, indexed by
their format. This meant that if a lecture contains more than one
resource with the same format (e.g. multiple pdf files), only the last
one gets downloaded. This commit fixes this problem.
We add the title of the resource to the filename of the downloaded
resource. However, for backward compatibility, we only do this for the
resources that we did not download because of the bug.
Merge pull request #160 from jonasdt/fix_resources

More sweetness from Jonas:

Fix downloading when multiple resources of a kind (say, PDFs and annotated PDFs)
are present in lectures.

This closes #65 and #72.
test: parsing: Use more descriptive variable names.

This way, I don't get nuts when I am reading the code some months from now.
test: parsing: Use Python's contexts for opening files.

This avoids resource leaks (open files).
README: Add documentation on issues with the -n option. Fixes #162.
README: Fix long lines.
Update .netrc example to include extra argument.
Merge pull request #164 from zqureshi/master

Update example using .netrc to include two dashes when used as
last argument before course name.

See comment https://github.com/jplehmann/coursera/issues/162#issuecomment-21069111

Fixes #162.
README: Add newer examples to avoid misleading people.

We had more than one issue filed where people thought that they only had to
specify the "base name" of the classes.

While this was true during the inception of Coursera, their naming of the
classes has changed during time and this makes the documentation clearer to
match the post-initial naming.
travis: Test also builds with pypy.
travis: Allow tests with pypy to fail.

Since people usually think of PyPy as a second-class citizen, we don't fail
in case one of our modules barfs with PyPy, but it is certainly a good thing
to test with it (and this author here would love to see PyPy become more
respected as it has the potential to make Python a high performance
language).
travis: Make nose tests verbose.

This way, we can easily see what works by names, instead of only seeing
little dots.
Update Python requirements in documentation.
Add required package six to the list of packages, as well as update instructions for requests package for Mac. In addition, add details on selecting python 2.7 for Mac Ports in order to get coursera-dl to find the required python libraries.
More informative error messages

I also think it should look up for the host coursera.org instead of
'coursera-dl'.
Merge pull request #176 from untitaker/master

Make some error messages wrt authentication more informative.
Add newline to the last line of file.
Merge pull request #177 from rdcastro/master

Update documentation of prerequistes for OS-specific installations.
README: Add warning about installation method of dependencies.
README: Try to fix markup.

[skip ci]