Stats page issue 288

Do we track any other data that we might want to display on this page?
This line failed for me in a system with no public studies, while realistically this will not happen, can you change it to fail graciously?

```
[E 141103 09:39:17 web:1228] Uncaught exception GET /stats/ (::1)
    HTTPRequest(protocol='http', host='localhost:8888', method='GET', uri='/stats/', version='HTTP/1.1', remote_ip='::1', headers={'Accept-Language': 'en-US,en;q=0.8,es;q=0.6,it;q=0.4', 'Accept-Encoding': 'gzip,deflate,sdch', 'Host': 'localhost:8888', 'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8', 'User-Agent': 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/38.0.2125.104 Safari/537.36', 'Connection': 'keep-alive', 'Referer': 'http://localhost:8888/', 'Cookie': 'skid="2|1:0|10:1412798868|4:skid|8:InRlc3Qi|e8640c782c441c27fd9d8eb8db2c00c034f5b3e35c471f400a2d1239eff5b8bf"; completed_survey_id="2|1:0|10:1412798974|19:completed_survey_id|24:NjNkN2MxMTA1ZDMyNDc3Nw==|f76072894e51c50ddedcd7922242abdb17b8c5f7838d5ddd6da84eaef42e0855"; _ga=GA1.1.700068573.1412281734; user=ImRlbW9AbWljcm9iaW8ubWUi|1415032752|d772fa15c9c1fc62940fd6970cb533eaee24bda0'})
    Traceback (most recent call last):
      File "/Users/yoshikivazquezbaeza/.virtualenvs/qiita/lib/python2.7/site-packages/tornado/web.py", line 1144, in _when_complete
        if result.result() is not None:
      File "/Users/yoshikivazquezbaeza/.virtualenvs/qiita/lib/python2.7/site-packages/tornado/concurrent.py", line 129, in result
        raise_exc_info(self.__exc_info)
      File "/Users/yoshikivazquezbaeza/.virtualenvs/qiita/lib/python2.7/site-packages/tornado/gen.py", line 221, in wrapper
        runner.run()
      File "/Users/yoshikivazquezbaeza/.virtualenvs/qiita/lib/python2.7/site-packages/tornado/gen.py", line 507, in run
        yielded = self.gen.send(next)
      File "/Users/yoshikivazquezbaeza/git_sw/qiita/qiita_pet/handlers/stats.py", line 63, in get
        study = Study(choice(Study.get_public()))
      File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/random.py", line 273, in choice
        return seq[int(self.random() * len(seq))]  # raises IndexError if seq is empty
    IndexError: list index out of range
[E 141103 09:39:17 web:1635] 500 GET /stats/ (::1) 26.36ms
```