HttpUtils.getLastModified(): Disconnect the connection when finished.

When using StrictMode we get this exception if we don't explicitly
disconnect the HttpConnection:
5893-5903/com.murrayc.galaxyzoo.app E/StrictMode﹕ A resource was acquired at attached stack trace but never released. See java.io.Closeable for information on avoiding resource leaks.
    java.lang.Throwable: Explicit termination method 'end' not called
ItemsContentProvider: Add some more infor logging.

This seems to show that the UI is not responsive when lots of
FileCacheAsyncTask.doInBackground() is happening, presumably
because of the writing to file storage.