Added state sub generator.

Added a yeoman sub generator to create phaser states that tie in with the generator-phaser framework layout.

The sub gen can be invoked as:
  `yo phaser:state`
and the sub gen will then ask for the name of the new state to create. 

The index.html and the main.js templates had to be updated to allow adding new references to the created state javascript files. There may be a more elegant way of accomplishing this.

Finally, I have not included unit tests for this code as I am unfamiliar with the unit testing framework.