Enhancements for SageMath Cloud

I've added a fix to the existing `iplot()` function to make it work like in the IPython notebook. (I've also fixed the quite ugly line for the `<iframe ... >`

The other fix is for encoding "native" Sage number types to JSON (They are implicitly created when numbers like `42` or `4.2` are entered in the Sage Notebook.)

To test it, install it in a project at https://cloud.sagemath.com