add support for Sotheby's

[Siddhant]

Example URL:
http://www.sothebys.com/content/sothebys/en/news-video/videos/2015/05/seven-artist-records-in-sothebys-contemporary-art-evening-auction.html

~ $
youtube-dl --verbose "http://www.sothebys.com/content/sothebys/en/news-video/videos/2015/05/seven-artist-records-in-sothebys-contemporary-art-evening-auction.html"
[debug] System config: []
[debug] User config: []
[debug] Command-line args: [u'--verbose', u'http://www.sothebys.com/content/sothebys/en/news-video/videos/2015/05/seven-artist-records-in-sothebys-contemporary-art-evening-auction.html']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2015.05.10
[debug] Python version 2.7.8 - Linux-3.16.0-37-generic-x86_64-with-Ubuntu-14.10-utopic
[debug] exe versions: avconv 11-6, avprobe 11-6
[debug] Proxy map: {}
[generic] seven-artist-records-in-sothebys-contemporary-art-evening-auction: Requesting header
WARNING: Falling back on generic information extractor.
[generic] seven-artist-records-in-sothebys-contemporary-art-evening-auction: Downloading webpage
[generic] seven-artist-records-in-sothebys-contemporary-art-evening-auction: Extracting information
ERROR: Unsupported URL: http://www.sothebys.com/content/sothebys/en/news-video/videos/2015/05/seven-artist-records-in-sothebys-contemporary-art-evening-auction.html
Traceback (most recent call last):
File "/usr/local/bin/youtube-dl/youtube_dl/extractor/generic.py", line 937, in _real_extract
doc = parse_xml(webpage)
File "/usr/local/bin/youtube-dl/youtube_dl/utils.py", line 1558, in parse_xml
tree = xml.etree.ElementTree.XML(s.encode('utf-8'), **kwargs)
File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1300, in XML
parser.feed(text)
File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1642, in feed
self._raiseerror(v)
File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1506, in _raiseerror
raise err
ParseError: not well-formed (invalid token): line 73, column 6986
Traceback (most recent call last):
File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 650, in extract_info
ie_result = ie.extract(url)
File "/usr/local/bin/youtube-dl/youtube_dl/extractor/common.py", line 273, in extract
return self._real_extract(url)
File "/usr/local/bin/youtube-dl/youtube_dl/extractor/generic.py", line 1467, in _real_extract
raise UnsupportedError(url)
UnsupportedError: Unsupported URL: http://www.sothebys.com/content/sothebys/en/news-video/videos/2015/05/seven-artist-records-in-sothebys-contemporary-art-evening-auction.html