Adds Auth0Widget.version property that contains the current auth0-widget 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 51 30](https://cloud.githubusercontent.com/assets/419703/3159144/1020ad30-eb0d-11e3-9953-b40b4e61c6f6.png)