Added requirements file
Updated documentation for installing requirements
Typo fix
Merge pull request #40 from azizmb/requirements

Added requirements.txt for pip installations
improved support for downloading several classes at once

- "at once", meaning one after the other
- removed --add-class, in favor of positional arguments
  - syntax is now: <script> ... <class1> [<class2> ...]
- removed exit() when class not found, instead throwing a custom
  exception which is handled in the top-level loop
Add ability to use aria2c for downloads.
Merge pull request #42 from rbrito/features/use-aria2

Add ability to use aria2c for downloads.
Prompt for password if not given in command line or .netrc
Update password prompt to be clearly not email related
Merge pull request #44 from mcarpenter/password

Prompt for password if not given in command line or .netrc
Use the logging module to control verboseness of program.
Remove commented out code from the program.
Reduce slightly the verboseness of aria2.
Move class definition before all functions.
Follow PEP-8 and add more spaces between functions.
Declutter a tiny bit by removing unnecessary parentheses.
Break (some) long lines.

There is more work to do, but that's for later.
Remove comment that local page option is for debugging.

It is also useful to download videos that are listed in preview pages, for
example.
Add some blanks for legibility.

We also break one if in two lines for happiness of static analysis tools.
Add some docstrings to some functions.

We may, potentially, use these to create doctests in the future.