relaxed restrictions on OAuth callbacks

src/main/java/org/scribe/builder/ServiceBuilder.java

@@ -84,7 +84,7 @@ public class ServiceBuilder
    */
   public ServiceBuilder callback(String callback)
   {
-    Preconditions.checkValidOAuthCallback(callback, "Callback must be a valid URL or 'oob'");
+    Preconditions.checkNotNull(callback, "Callback can't be null");
     this.callback = callback;
     return this;
   }

src/test/java/org/scribe/builder/ServiceBuilderTest.java

@@ -46,9 +46,9 @@ public class ServiceBuilderTest
   }
 
   @Test(expected=IllegalArgumentException.class)
-  public void shouldNotAcceptAnInvalidUrlAsCallback()
+  public void shouldNotAcceptNullAsCallback()
   {
-    builder.provider(ApiMock.class).apiKey("key").apiSecret("secret").callback("example.com").build(); 
+    builder.provider(ApiMock.class).apiKey("key").apiSecret("secret").callback(null).build(); 
   }
 
   @Test