Singleton not thread safe

[iamareebjamal]

The singleton pattern is prone to synchronization errors and is not thread-safe. If multiple thread mutate the state of Push Raven, it will lead to an inconsistent state. The library is really great and I would like to contribute in it after some discussion over design patterns are done

I think we can achieve simpler syntax with production level design with minimal impact on current API

[Raudius]

I am open to any suggestions, in fact the singleton implementation came about after issue #2

[iamareebjamal]

Sure. I will add the proposed changes here soon