Improve some log messages.
ItemsContentProvider: Extract a general httpGetRequest method.
ItemsContentProvider: Moved httpGetRequest() to HttpUtils.
ItemsContentProvider: Use HttpURLConnection for caching files too.

This removes the last use of HttpClient.
Remove some unused imports.
ItemsContentProvider: Make sure that we close some streams.

However, I wonder if Java 7's try-with-resources can be used to
close streams even if no exceptions are expected, for instance if
we just get the wrong response code after calling getInputStream().
IconsCache: getFileContents: Use try-with-resources to close streams.
Singleton: Constructor: Use try-with-resources.
More use of try-with-resources to close streams.
DecisionTreeTest.setup(): Close the stream.
Use try-with-resources with OutputStreams too.
IconsCache: More use of try-with-resources.