Added support for dash/underscore in custom URLs

src/main/java/org/scribe/utils/Preconditions.java

@@ -7,7 +7,7 @@ import org.scribe.model.OAuthConstants;
 public class Preconditions
 {
   private static final String DEFAULT_MESSAGE = "Received an invalid parameter";
-  private static final Pattern URL_PATTERN = Pattern.compile("[a-zA-Z]+://\\S+");
+  private static final Pattern URL_PATTERN = Pattern.compile("[a-zA-Z_-]+://\\S+");
 
   public static void checkNotNull(Object object, String errorMsg)
   {

src/test/java/org/scribe/utils/PreconditionsTest.java

@@ -55,13 +55,13 @@ public class PreconditionsTest
   
   public void shouldAllowNonStandarProtocolsForAndroid()
   {
-    Preconditions.checkValidUrl("myUrl://www.example.com", ERROR_MSG);
+    Preconditions.checkValidUrl("x-url-custom://www.example.com", ERROR_MSG);
   }
   
   @Test(expected = IllegalArgumentException.class)
   public void shouldNotAllowStrangeProtocolNames()
   {
-    Preconditions.checkValidUrl("_weird*://www.example.com", ERROR_MSG);
+    Preconditions.checkValidUrl("$weird*://www.example.com", ERROR_MSG);
   }
   
   public void shouldAllowOutOfBandAsValidCallbackValue()