added manifest.webapp and 512px icon for Firefox OS

public/manifest.webapp

+{
+    "name": "RocketChat",
+    "description": "The Ultimate Open Source WebChat Platform",
+    "launch_path": "/",
+    "icons": {
+        "128": "/images/logo/favicon-128x128.png",
+        "512": "/images/logo/favicon-512x512.png"
+    },
+    "developer": {
+        "name": "Rocket.Chat",
+        "url": "https://rocket.chat/"
+    },
+    "fullscreen": "true",
+    "chrome": {
+        "navigation": false
+    }
+}