Fix to allow not expected resource types

Added a condition to get_file_ext function to allow not expected
resource types, like the ones found in Computational Investing Part I
course

Fixes #6 issue
- Updated CSS selector class names to reflect updates to Coursera
- Fixed file names for lectures named with &nbsp; and &quot; fixed in escape_name()
-- Shifted call of escape_name() to precede strip()  (avoids unnecessary spaces)
- Includes unpulled patch by jmorenoamor (https://github.com/dmotitsk/coursera/pull/7)
Some minor improvements of indentation