Move Twitter, Mobile and Ads Demo to Node.js - fixes #34

Node.js App with demos is created at Malhar/webdemos
Malhar/contrib/src/main/apps is removed.

Fixes the following:
#32 Common Assets for the Demos
#33 Ads Dimensions Demo - Common Template
#34 Move Twitter, Mobile and Ads Demo to Node.js
#37 Move Twitter Demo to Node.js
#38 Move Mobile Demo to Node.js
#40 Move Ads Dimensions Demo to Node.js
#41 Web Demos - License Headers