Added Flask-Demo and Flask AppBuilder tutorial

Flask-Demo/app.py

@@ -0,0 +1,17 @@
+from flask import Flask
+
+app = Flask(__name__)
+
+
+@app.route("/")
+def hello_world():
+    return "<p>Hello, world!</p>"
+
+
+@app.route("/haha")
+def second_example():
+    return "<p>Go dukes!</p>"
+
+
+if __name__ == "__main__":
+    app.run(debug=True)