Approval workflow with Azure Logic Apps, Office 365 and SharePoint
You should consider Azure Logic Apps for future development because It makes it easier to build web, mobile, and integration apps. It is workflow driven system where actions and conditions can be nicely combined to form flow of a business process.
Build in SaaS connectors are provided to many of the Microsoft technologies including the Office 365 stack, Microsoft SQL, OneDive, Dynamics and BizTalk services as well as providers like Salesforce, Dropbox, SAP, Github, Facebook, Tweeter, Google Drive and many others are added even now.
For expert scenarios a Logic App can be extended by custom code trough API APP or Azure Functions created with your preferred programming language (C#, Node.js, Python or other).
The Azure portal
Get started: http://aka.ms/logicappsquickstart
Get started: https://github.com/logicappsio
User voice: https://feedback.azure.com/forums/287593-logic-apps?filter=top&page=1