ES Standard Adoption

New Release Candidate: 1.5.0 🎉

Problem :

• The csrf tokens created did not follow the singleton pattern.
• Occasional use of strict mode keywords made the package usage difficult in ES6 compliant environments

Solution :

• csrf token instance created at the time of instantiating OAuthClient. Singleton JS design pattern adopted.
• Adopted ES6 standardization
• ESLint enabled

Issues Fixed :

#4
#30