contacts/conversations: adds saving conversation to json

- adds saving contact specific conversation to a json file

- loads the json file into the conversation object when loading
  the contacts list

- generates a UID for the contact to avoid name collisions, which
  is stored in the contact data file

Change-Id: I8a7b757d09b270e65c4d264805934faf8466c09f
Tuleap: #980
diff --git a/RingD.cpp b/RingD.cpp
index 6da6729..c69845c 100644
--- a/RingD.cpp
+++ b/RingD.cpp
@@ -90,6 +90,10 @@
     /* conversation */

     if (sent) {

         contact->_conversation->addMessage(""/* date not yet used*/, MSG_FROM_ME, message);

+

+        /* save contacts conversation to disk */

+        contact->saveConversationToFile();

+

     } else {

         WNG_("message not sent, see daemon outputs");

     }