Added bunch of javadocs and restricted some visibility.

No functional changes

src/main/java/org/scribe/model/Verifier.java

@@ -1,10 +1,19 @@
 package org.scribe.model;
 
+/**
+ * Represents an OAuth verifier code.
+ * 
+ * @author Pablo Fernandez
+ */
 public class Verifier
 {
-
   private final String value;
 
+  /**
+   * Default constructor.
+   * 
+   * @param value verifier value
+   */
   public Verifier(String value)
   {
     this.value = value;