Remove useless prefix of AUTH_URL.

The `NEW_` is not new anymore and anybody reading the code would be
wondering about the historic/legacy reasons why the prefix is there.

For those really curious, the log history of the project is there.
Try to make a long line at least a bit more readable.
Make the script more resilient to server errors.

In particular, this gets rid of stack traces when encountering pesky HTTP
500 errors while downloading a batch of classes---if one class gives errors
the exception would prevent other classes from being downloaded, which is
clearly suboptimal.
Minor docstrings work.
Use raw strings for regular expressions.
Remove unused parameter for the `get_page` function.
Adding retry feature for timeout errors.
Added support for courses in Preview mode
Indenting fixes
Merge pull request #90 from vojnovski/master

Added support for courses in Preview mode that have been taught previously and are about to reopen
Merge pull request #86 from romeluko/master

Retry feature on timeout
Remove some trailing whitespace.
test: Fix PEP-8 compliance.
test: Remove unused module.
test: Sort stdlib imports and separate local import from the former.
test: Remove setUp method of tests.

If there is a better way of introducing other tests (in particular, the
preview tests), it would be nice to know.
test: Add unit test for preview pages.

There's an unfortunate fact regarding the way that we treat preview pages,
which has another undesired side-effect.

* The unfortunate fact is that we call BeautifulSoup for each preview
  video/link on the preview page.
* The side effect is that we end up having to need network access to in the
  unittest for the previews.

Actually, a consequence of the last point is that for a class that has a lot
of lectures (say, a long course), the test takes a lot of time.
test: Disable preview-related tests, as they are too costly.

We may even get a warning or blacklisting or whatever from coursera, since
anybody who proposes a pull request to us will have their contributions
tested and this, in turn, can end up being like a DoS attack against
coursera (OK, I am exaggerating here).

But the fact is that I don't like that. We do need more tests, though.
Fix the script dying when grabbing non-existant resources.
Add unittest for links to Wikipedia.

Note that we are always assuming that links point towards places where we
can aways get things from, which is false.

Additionally, we are constructing incorrect URLs for external sites in the
case of no videos, which is a bug and causes extra (and spurious) network
access.