Added bunch of javadocs and restricted some visibility.

No functional changes

src/main/java/org/scribe/exceptions/OAuthSignatureException.java

@@ -1,9 +1,20 @@
 package org.scribe.exceptions;
 
+/**
+ * Specialized exception that represents a problem in the signature
+ * 
+ * @author Pablo Fernandez
+ */
 public class OAuthSignatureException extends OAuthException
 {
   private static final long serialVersionUID = 1L;
   
+  /**
+   * Default constructor
+   * 
+   * @param stringToSign plain string that gets signed (HMAC-SHA, etc)
+   * @param e original exception
+   */
   public OAuthSignatureException(String stringToSign, Exception e)
   {
     super("Error while signing string: " + stringToSign, e);