Change SMS/Voice code delivery priorities.

1) Honor a twilio.international configuration boolean that
   specifies whether to use twilio for international destinations.

2) If a nexmo configuration is specified, and Twilio fails to
   deliver, fall back and attempt delivery again with nexmo.

src/main/java/org/whispersystems/textsecuregcm/sms/SmsSender.java

@@ -0,0 +1,84 @@
+/**
+ * Copyright (C) 2013 Open WhisperSystems
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.whispersystems.textsecuregcm.sms;
+
+
+import com.google.common.base.Optional;
+import com.twilio.sdk.TwilioRestException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+
+public class SmsSender {
+
+  static final String SMS_VERIFICATION_TEXT = "Your TextSecure verification code: ";
+  static final String VOX_VERIFICATION_TEXT = "Your TextSecure verification code is: ";
+
+  private final Logger logger = LoggerFactory.getLogger(SmsSender.class);
+
+  private final TwilioSmsSender          twilioSender;
+  private final Optional<NexmoSmsSender> nexmoSender;
+  private final boolean                  isTwilioInternational;
+
+  public SmsSender(TwilioSmsSender twilioSender,
+                   Optional<NexmoSmsSender> nexmoSender,
+                   boolean isTwilioInternational)
+  {
+    this.isTwilioInternational = isTwilioInternational;
+    this.twilioSender          = twilioSender;
+    this.nexmoSender           = nexmoSender;
+  }
+
+  public void deliverSmsVerification(String destination, String verificationCode)
+      throws IOException
+  {
+    if (!isTwilioDestination(destination) && nexmoSender.isPresent()) {
+      nexmoSender.get().deliverSmsVerification(destination, verificationCode);
+    } else {
+      try {
+        twilioSender.deliverSmsVerification(destination, verificationCode);
+      } catch (TwilioRestException e) {
+        logger.info("Twilio SMS Fallback", e);
+        if (nexmoSender.isPresent()) {
+          nexmoSender.get().deliverSmsVerification(destination, verificationCode);
+        }
+      }
+    }
+  }
+
+  public void deliverVoxVerification(String destination, String verificationCode)
+      throws IOException
+  {
+    if (!isTwilioDestination(destination) && nexmoSender.isPresent()) {
+      nexmoSender.get().deliverVoxVerification(destination, verificationCode);
+    } else {
+      try {
+        twilioSender.deliverVoxVerification(destination, verificationCode);
+      } catch (TwilioRestException e) {
+        logger.info("Twilio Vox Fallback", e);
+        if (nexmoSender.isPresent()) {
+          nexmoSender.get().deliverVoxVerification(destination, verificationCode);
+        }
+      }
+    }
+  }
+
+  private boolean isTwilioDestination(String number) {
+    return isTwilioInternational || number.length() == 12 && number.startsWith("+1");
+  }
+}