Milestone v4.0.0

• Drop gulp dependency
• Drop coffee script support
• Cross platform CI (Linux and Windows)
• Drop support for Node v0.12 or below
• Codebase
  • ES 2015/2017, either use Node v4 base or babel build
  • Migrate codebase
  • Migrate tests
  • Use lodash/fp or ramda
• Sequelize v4 Support moving model.associate outside classMethods due to sequelize@4.0.0 #470
• Better documentation
  • Contribution Docs
  • Section in https://docs.sequelizejs.com
  • Explain available QueryGenerator functions like bulkInsert etc and how to properly use them
  • FAQs
• Features / Support
  • db:create / db:drop
  • Proper error codes
  • Association support
  • Atomic migrations (using transactions, as far as database supports it)
  • TypeScript support (needs to be community contributed, after v4 is released)