Changed 'descriptor.collector' default value to DescriptorIndexCollector.

Implements task-20404.

CHANGELOG.md

 # Changes in version 1.?.? - 2016-1?-??
 
+ * Major changes
+   - Make the DescriptorCollector implementation that uses CollecTor's
+     index.json file to determine which descriptor files to fetch the
+     new default.  Applications must provide gson-2.2.4.jar or higher
+     as dependency.
+
  * Minor changes
    - Avoid running into an IOException and logging a warning for it.
 

src/main/java/org/torproject/descriptor/DescriptorSourceFactory.java

@@ -72,7 +72,7 @@ public final class DescriptorSourceFactory {
    * @since 1.0.0
    */
   public static final String COLLECTOR_DEFAULT =
-      "org.torproject.descriptor.impl.DescriptorCollectorImpl";
+      "org.torproject.descriptor.index.DescriptorIndexCollector";
 
   /**
    * Property name for overriding the implementation of the

src/test/java/org/torproject/descriptor/DescriptorSourceFactoryTest.java

@@ -9,10 +9,10 @@ import org.torproject.descriptor.DescriptorDownloader;
 import org.torproject.descriptor.DescriptorParser;
 import org.torproject.descriptor.DescriptorReader;
 import org.torproject.descriptor.ImplementationNotAccessibleException;
-import org.torproject.descriptor.impl.DescriptorCollectorImpl;
 import org.torproject.descriptor.impl.DescriptorDownloaderImpl;
 import org.torproject.descriptor.impl.DescriptorParserImpl;
 import org.torproject.descriptor.impl.DescriptorReaderImpl;
+import org.torproject.descriptor.index.DescriptorIndexCollector;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNull;
@@ -35,7 +35,7 @@ public class DescriptorSourceFactoryTest {
   public void testDefaults() {
     setProperties(defaults);
     DescriptorCollector dc = DescriptorSourceFactory.createDescriptorCollector();
-    assertTrue(dc instanceof DescriptorCollectorImpl);
+    assertTrue(dc instanceof DescriptorIndexCollector);
     DescriptorDownloader dd = DescriptorSourceFactory.createDescriptorDownloader();
     assertTrue(dd instanceof DescriptorDownloaderImpl);
     DescriptorParser dp = DescriptorSourceFactory.createDescriptorParser();
@@ -101,7 +101,7 @@ public class DescriptorSourceFactoryTest {
 
 }
 
-class DummyCollectorImplementation extends DescriptorCollectorImpl {
+class DummyCollectorImplementation extends DescriptorIndexCollector {
   static int count;
   public DummyCollectorImplementation() {
     count++;