DataServer: downloadJsonObject -> downloadJsonText

The convertion of the downloaded JSON text to a JSONObject now occurs in
getJsonObject().
DataServer: statically initialize the HttpClient
DataServer: use HttpURLConnection

It is recommended to use HttpURLConnection instead of DefaultHttpClient.

See:
- http://developer.android.com/reference/org/apache/http/impl/client/DefaultHttpClient.html
- http://android-developers.blogspot.fr/2011/09/androids-http-clients.html
DataServer: add method to get JSON from data.bnf.fr
Extract code from ImageLoader to ImageHelper
Add DataBnfFrImageLoader
Add DataBnfFrImageLoaderCallbacks
ViewObjectActivity: load image from data.bnf.fr
Comment the image-related code that cannot be used

See the comment for more explanation.