Fix to allow not expected resource types in some courses

Added a condition to get_file_ext function to allow not expected
resource types, like the ones found in Computational Investing Part I
course.

Example:

    <a target="_new" href="https://class.coursera.org/compinvesting1-2012-001/quiz/start?quiz_id=2" data-placement="above" rel="twipsy" title="Quiz">
    <i class="icon-edit resource"></i>
    <div class="hidden">Quiz for Sample Lecture</div>
</a>

Fixes #6 issue