completed core unit test for parsing the syllabus html

- counts the number of sections, lectures, resources (of all types) and
  specifically mp4 files found
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