slash commands: Add /ping command (via zcommand).

This adds a /ping command that will be useful for users
to see what the round trip to the Zulip server is (including
only a tiny bit of actual server time to basically give a
200).

It also introduce the "/zcommand" endpoint and zcommand.js
module.

static/js/zcommand.js

@@ -0,0 +1,82 @@
+var zcommand = (function () {
+
+var exports = {};
+
+/*
+
+What in the heck is a zcommand?
+
+    A zcommand is basically a specific type of slash
+    command where the client does almost no work and
+    the server just does something pretty simple like
+    flip a setting.
+
+    The first zcommand we wrote is for "/ping", and
+    the server just responds with a 200 for that.
+
+    Not all slash commands use zcommand under the hood.
+    For more exotic things like /poll see submessage.js
+    and widgetize.js
+
+*/
+
+exports.send = function (opts) {
+    var command = opts.command;
+    var on_success = opts.on_success;
+    var data = {
+        command: command,
+    };
+
+    channel.post({
+        url: '/json/zcommand',
+        data: data,
+        success: function (data) {
+            on_success(data);
+        },
+        error: function () {
+            exports.tell_user('server did not respond');
+        },
+    });
+};
+
+exports.tell_user = function (msg) {
+    // This is a bit hacky, but we don't have a super easy API now
+    // for just telling users stuff.
+    $('#compose-send-status').removeClass(common.status_classes)
+        .addClass('alert-error')
+        .stop(true).fadeTo(0, 1);
+    $('#compose-error-msg').text(msg);
+};
+
+exports.process = function (message_content) {
+
+    var content = message_content.trim();
+
+    if (content === '/ping') {
+        var start_time = new Date();
+
+        exports.send({
+            command: 'ping',
+            on_success: function () {
+                var end_time = new Date();
+                var diff = end_time - start_time;
+                diff = Math.round(diff);
+                var msg = "ping time: " + diff + "ms";
+                exports.tell_user(msg);
+            },
+        });
+        return true;
+    }
+
+    // It is incredibly important here to return false
+    // if we don't see an actual zcommand, so that compose.js
+    // knows this is a normal message.
+    return false;
+};
+
+return exports;
+}());
+
+if (typeof module !== 'undefined') {
+    module.exports = zcommand;
+}