Added bunch of javadocs and restricted some visibility.

No functional changes

src/main/java/org/scribe/services/SignatureService.java

@@ -1,8 +1,28 @@
 package org.scribe.services;
 
+/**
+ * Signs a base string, returning the OAuth signature
+ * 
+ * @author Pablo Fernandez
+ *
+ */
 public interface SignatureService
 {
+  /**
+   * Returns the signature
+   * 
+   * @param baseString url-encoded string to sign
+   * @param apiSecret api secret for your app
+   * @param tokenSecret token secret (empty string for the request token step)
+   * 
+   * @return signature
+   */
   public String getSignature(String baseString, String apiSecret, String tokenSecret);
 
+  /**
+   * Returns the signature method/algorithm
+   * 
+   * @return
+   */
   public String getSignatureMethod();
 }