Added Payment API

Jira-issue: WP-651
Added DemoWallet Payment API Implementation

Jira-Issue: WP-651
Fix payment API build errors
Made it more resilient to missing demowallet app.

If the DemoWallet application is missing, it won't crash webinos.
Updated Readme to add special notes for vs 2012

Vs 2012 need extra parameters passed in the npm command.
Merge pull request #42 from andreasbotsikas/WP-651

Added Payment API