Set template_path for ClientListHandler.

The new Tornado `ClientListHandler` does not set a `template_path` but relies on implicit default (module) directory for resolving the `data/client.html` template. Now, if an HTTP extension sets its own `template_path`, this seems to override the `ClientListHandler`'s template path, too:

```
Traceback (most recent call last):
  File "/home/tkemmer/lib/python2.7/site-packages/tornado-3.2.2-py2.7-linux-x86_64.egg/tornado/web.py", line 1346, in _when_complete
    callback()
  File "/home/tkemmer/lib/python2.7/site-packages/tornado-3.2.2-py2.7-linux-x86_64.egg/tornado/web.py", line 1367, in _execute_method
    self._when_complete(method(*self.path_args, **self.path_kwargs),
  File "/home/tkemmer/src/mopidy/mopidy/http/handlers.py", line 165, in get
    self.render('data/clients.html', apps=sorted(list(names)))
  File "/home/tkemmer/lib/python2.7/site-packages/tornado-3.2.2-py2.7-linux-x86_64.egg/tornado/web.py", line 666, in render
    html = self.render_string(template_name, **kwargs)
  File "/home/tkemmer/lib/python2.7/site-packages/tornado-3.2.2-py2.7-linux-x86_64.egg/tornado/web.py", line 770, in render_string
    t = loader.load(template_name)
  File "/home/tkemmer/lib/python2.7/site-packages/tornado-3.2.2-py2.7-linux-x86_64.egg/tornado/template.py", line 343, in load
    self.templates[name] = self._create_template(name)
  File "/home/tkemmer/lib/python2.7/site-packages/tornado-3.2.2-py2.7-linux-x86_64.egg/tornado/template.py", line 370, in _create_template
    f = open(path, "rb")
IOError: [Errno 2] No such file or directory: u'/home/tkemmer/src/mopidy-http-kuechenradio/mopidy_http_kuechenradio/data/template/data/clients.html'
```

Explicitly setting the  `template_path` apparently solves this. Don't know if this is a Tornado bug, but *explicit is better than implicit* anyway.
Sorry, completely missed that. Yes, overriding `RequestHandler.get_template_path()` seems to have the same effect, i.e. the ClientListHandler and my extension using different template directories. I'm still a little confused how this actually gets handled by Tornado; if `self.settings` overrides all extensions, wouldn't the ClientListHandler be affected as well? However, since other authors of Mopidy HTTP extensions who are as blissfully ignorant of Tornado's ways may run into the same problem, maybe a short remark in the API docs/FAQs would make sense?