Added plaintext signature for Yammer Api and fixed fucked up indentation

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

@@ -0,0 +1,43 @@
+package org.scribe.services;
+
+import org.scribe.exceptions.*;
+import org.scribe.utils.*;
+
+/**
+ * plaintext implementation of {@SignatureService}
+ *
+ * @author Pablo Fernandez
+ *
+ */
+public class PlaintextSignatureService implements SignatureService
+{
+  private static final String EMPTY_STRING = "";
+  private static final String CARRIAGE_RETURN = "\r\n";
+  private static final String UTF8 = "UTF-8";
+  private static final String METHOD = "plaintext";
+
+  /**
+   * {@inheritDoc}
+   */
+  public String getSignature(String baseString, String apiSecret, String tokenSecret)
+  {
+    try
+    {
+      Preconditions.checkEmptyString(apiSecret, "Api secret cant be null or empty string");
+      return URLUtils.percentEncode(apiSecret) + '&' + URLUtils.percentEncode(tokenSecret);
+    }
+    catch (Exception e)
+    {
+      throw new OAuthSignatureException(baseString, e);
+    }
+  }
+
+  /**
+   * {@inheritDoc}
+   */
+  public String getSignatureMethod()
+  {
+    return METHOD;
+  }
+}
+