Add "last resort" PreKey generation to KeyHelper.

libaxolotl/src/main/java/org/whispersystems/libaxolotl/util/KeyHelper.java

@@ -74,4 +74,15 @@ public class KeyHelper {
     return results;
   }
 
+  /**
+   * Generate the last resort PreKey.  Clients should do this only once, at install
+   * time, and durably store it for the length of the install.
+   *
+   * @return the generated last resort PreKeyRecord.
+   */
+  public static PreKeyRecord generateLastResortPreKey() {
+    ECKeyPair keyPair = Curve.generateKeyPair(true);
+    return new PreKeyRecord(Medium.MAX_VALUE, keyPair);
+  }
+
 }