Extract supplement links from the html text that contains <a> tags
        with href attribute.

        @param text: HTML text.
        @type text: str

        @return: Dictionary with supplement links grouped by extension.
        @rtype: {
            '<extension1>': [
                ('<link1>', '<title1>'),
                ('<link2>', '<title2')
            ],
            'extension2': [
                ('<link3>', '<title3>'),
                ('<link4>', '<title4>')
            ]
        }

def _extract_links_from_a_tags_in_text(self, text):
        """
        Extract supplement links from the html text that contains <a> tags
        with href attribute.

        @param text: HTML text.
        @type text: str

        @return: Dictionary with supplement links grouped by extension.
        @rtype: {
            '<extension1>': [
                ('<link1>', '<title1>'),
                ('<link2>', '<title2')
            ],
            'extension2': [
                ('<link3>', '<title3>'),
                ('<link4>', '<title4>')
            ]
        }
        """
        soup = BeautifulSoup(text)
        links = [item['href'].strip()
                 for item in soup.find_all('a') if 'href' in item.attrs]
        links = sorted(list(set(links)))
        supplement_links = {}

        for link in links:
            filename, extension = os.path.splitext(clean_url(link))
            # Some courses put links to sites in supplement section, e.g.:
            # http://pandas.pydata.org/
            if extension is '':
                continue

            # Make lowercase and cut the leading/trailing dot
            extension = clean_filename(
                extension.lower().strip('.').strip(),
                self._unrestricted_filenames)
            basename = clean_filename(
                os.path.basename(filename),
                self._unrestricted_filenames)
            if extension not in supplement_links:
                supplement_links[extension] = []
            # Putting basename into the second slot of the tuple is important
            # because that will allow to download many supplements within a
            # single lecture, e.g.:
            # 01_slides-presented-in-this-module.pdf
            # 01_slides-presented-in-this-module_Dalal-cvpr05.pdf
            # 01_slides-presented-in-this-module_LM-3dtexton.pdf
            supplement_links[extension].append((link, basename))

        return supplement_links