test: Change syntax for greater compatibility with Python 3.

We still have some way to go, but it is getting shorter.
Fix example in README

The user should always provide a username or .netrc file.
Add credentials module

This module manages the credential information (netrc, passwords, etc).
Add credentials.get_credentials

The function returns valid username and password. It should be called
with the corresponding values in the parsed args.
Use get_credentials in coursera_dl
Merge pull request #128 from jonasdt/credentials

Make aquisition of locally-provided credentials a separate module.
coursera: Move local modules to the end, as per PEP-8.

The [rule for imports][0] is:

* standard library modules.
* third-party modules.
* local modules.

[0]: http://www.python.org/dev/peps/pep-0008/#imports
coursera: credentials: Move lambda function to be a proper function.

This helps us with the code being self explanatory (and, of course, with
comments transformed into docstrings).
coursera: credentials: Try to make the code more explicit.

Hopefully, slightly more readable, but I doubt it. Besides, this really
belongs in the standard library, not in our code.

Windows users will probably cry if I did something stupid, but we can just
revert this, in that case.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
coursera: credentials: Convert long comment into proper documentation.