Fixed some strange errors on brython to make brythonmagic works fine on the Jupyter notebook.
- Added some functions to easily load javascript libraries that can be used by brython in the notebook:
- `load_brython_stable`: will load the latest _brython_dist.js_ stable release from [http://www.brython.info/src/brython_dist.js](http://www.brython.info/src/brython_dist.js).
- `load_brython_dev`: will load the latest _brython_dist.js_ development version from github using rawgit.
- `load_js_lib`: Allows you to load javascript libs using a string indicating the url.
- Updated the code to make it work with latest jupyter releases.