Adds Auth0.version property that contains the current auth0.js version

Adding `Auth0.version` property that informs the user the current Auth0.js version that is being used. This is both useful for debugging and support given that the current running version is unknown.

![screen shot 2014-06-03 at 12 43 24](https://cloud.githubusercontent.com/assets/419703/3159056/ea887b8a-eb0b-11e3-9cec-4227c41f85ea.png)