typeahead: Add recent_senders module.

In recent_senders module, added a data structure to hold timestamps of
users' latest message in a topic. Also added a function to compare 2
users based on above timestamp. Added a function to process messages for
the data structure and a call in add_message_metadata. Also added node
tests for insertion of data into recent_senders.senders.

frontend_tests/node_tests/recent_senders.js

@@ -0,0 +1,74 @@
+var assert = require('assert');
+var rs = require('js/recent_senders.js');
+var _ = require('node_modules/underscore/underscore.js');
+
+(function test_process_message_for_senders() {
+    var stream1 = 1;
+    var stream2 = 2;
+
+    var topic1 = "topic-1";
+    var topic2 = "topic-2";
+
+    var sender1 = 1;
+    var sender2 = 2;
+    var sender3 = 3;
+
+    // New stream
+    var message1 = {
+        stream_id: stream1,
+        timestamp: _.uniqueId(),
+        subject: topic1,
+        sender_id: sender1,
+    };
+    var message2 = {
+        stream_id: stream2,
+        timestamp: _.uniqueId(),
+        subject: topic1,
+        sender_id: sender2,
+    };
+    rs.process_message_for_senders(message1);
+    rs.process_message_for_senders(message2);
+
+    assert.equal(
+        rs.compare_by_recency({user_id: sender1}, {user_id: sender2}, stream1, topic1) < 0,
+        true);
+    assert.equal(
+        rs.compare_by_recency({user_id: sender1}, {user_id: sender2}, stream2, topic1) > 0,
+        true);
+
+    // New topic
+    var message3 = {
+        stream_id: stream1,
+        timestamp: _.uniqueId(),
+        subject: topic2,
+        sender_id: sender3,
+    };
+    rs.process_message_for_senders(message3);
+    assert.equal(
+        rs.compare_by_recency({user_id: sender3}, {user_id: sender2}, stream1, topic2) < 0,
+        true);
+
+    // New sender
+    var message4 = {
+        stream_id: stream1,
+        timestamp: _.uniqueId(),
+        subject: topic1,
+        sender_id: sender2,
+    };
+    rs.process_message_for_senders(message4);
+    assert.equal(
+        rs.compare_by_recency({user_id: sender1}, {user_id: sender2}, stream1, topic1) > 0,
+        true);
+
+    // More recent message
+    var message5 = {
+        stream_id: stream1,
+        timestamp: _.uniqueId(),
+        subject: topic1,
+        sender_id: sender1,
+    };
+    rs.process_message_for_senders(message5);
+    assert.equal(
+        rs.compare_by_recency({user_id: sender1}, {user_id: sender2}, stream1, topic1) < 0,
+        true);
+}());