Depend on the Volley library.
Correct some log output.
Use Volley for getting the JSON list of subjects and for getting the images.

The code is now a bit awkward, but it seems to have much better performance,
probably because it avoids doing too many simultaneous network requests.
Login and Upload are still using plain HttpURLConnection because they
are so simple.
strings.xml: About: Mention the use of the Volley library.
LoginActivity: Fix the crash due to a null ZooniverseClient.
ItemsContentProvider: Improve some log output.