Import the CurlAsyncHTTPClient if avail

Imports the `tornado.httpclient.CurlAsyncHTTPClient` if available, otherwise falls back to `tornado.httpclient.AsyncHTTPClient`.

Need to determine if the built with an asynchronous DNS resolver. From [the docs](http://tornado.readthedocs.org/en/latest/httpclient.html):

> It is highly recommended that your libcurl installation is built with asynchronous DNS resolver (threaded or c-ares), otherwise you may encounter various problems with request timeouts (for more information, see http://curl.haxx.se/libcurl/c/curl_easy_setopt.html#CURLOPTCONNECTTIMEOUTMS and comments in curl_httpclient.py).

According to the docs, it says to configure the CurlAsyncHTTPClient through configuration instead of the import that's in this PR:

```
AsyncHTTPClient.configure("tornado.curl_httpclient.CurlAsyncHTTPClient")
```

TODO:
* [ ] Add `libcurl4-openssl-dev` to the base packages once #28 is merged.