Port to Python3 and Sublime Text 3 API

Hi, thank you for this helpful plugin that fits very nicely with my workflow! I recently got a license for Sublime Text 3, so I ported the IPython integration to python 3 syntax and the new API. Maybe you want to pull it into a new branch in your repo. I tested the main functionalities, but I'm not really sure if I changed all the necessary places. However, it should not take much to fix any remaining bugs.

Thanks again!