[YoutubeDL] Escape non-ASCII characters in URLs

urllib chokes on URLs with non-ASCII characters (see http://bugs.python.org/issue3991) with something like this:
```
File "/usr/lib/python2.7/urllib2.py", line 404, in open
    response = self._open(req, data)
  File "/usr/lib/python2.7/urllib2.py", line 422, in _open
    '_open', req)
  File "/usr/lib/python2.7/urllib2.py", line 382, in _call_chain
    result = func(*args)
  File "/usr/lib/python2.7/urllib2.py", line 1214, in http_open
    return self.do_open(httplib.HTTPConnection, req)
  File "/usr/lib/python2.7/urllib2.py", line 1181, in do_open
    h.request(req.get_method(), req.get_selector(), req.data, headers)
  File "/usr/lib/python2.7/httplib.py", line 973, in request
    self._send_request(method, url, body, headers)
  File "/usr/lib/python2.7/httplib.py", line 1007, in _send_request
    self.endheaders(body)
  File "/usr/lib/python2.7/httplib.py", line 969, in endheaders
    self._send_output(message_body)
  File "/usr/lib/python2.7/httplib.py", line 829, in _send_output
    self.send(msg)
  File "/usr/lib/python2.7/httplib.py", line 805, in send
    self.sock.sendall(data)
  File "/usr/lib/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
UnicodeEncodeError: 'ascii' codec can't encode character u'\u0301' in position 57: ordinal not in range(128)
```
At least [telemb.py](https://github.com/rg3/youtube-dl/blob/f24e740bb8e40bcd12dea004ad44104d672b5884/youtube_dl/extractor/telemb.py#L26) and [ard.py](https://github.com/rg3/youtube-dl/blob/f24e740bb8e40bcd12dea004ad44104d672b5884/youtube_dl/extractor/ard.py#L53) (#3412) are affected.
This PR proposes generic solution to aforementioned issue by substituting the original URL of any request that goes through `YoutubeDL.urlopen` with the escaped URL.
`escape_path` looks like an undocumented method, I cannot find any reference to it in the official Python docs. What's wrong with `quote_plus`?