Bug Fixes and Webhook signature validation

1. Added a static method to validate webhook signature https://github.com/razorpay/razorpay-node/wiki#webhooks
2. Fixed Minor bugs in the package

Support for Plans, Subscriptions, Addons

Added support for more APIs

• More api methods for Virtual Accounts
• Plans API
• Subscriptions API
• Addons API

Version Bump

Changes

• Adds dist folder in the source code
• Version Bump

Support for Invoices

Invoices API with Testcases

• added Invoices api

• Invoices unit tests

• Updated readme and wiki

• Bumped the version to 1.5.0

Support for Virtual Accounts

Virtual accounts with test cases (#13)

* added virtual accounts api

* Virtual Accounts unit tests

* Added promise polyfill

* Updated readme and wiki

* Bumped the version to 1.4.0