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.