Make IPython Magics work

Currently, we don't support IPython Magics at all from noteboks. We initially supported them, but removed them because importing IPython was causing a massive startup time lag (>1s) that seemed pretty bad.

Ideally, I'd like to be able to do the following:
1. If IPython is already loaded (you are already in a notebook), support magics by default
2. If IPython is *not* loaded, and there are no magics in the imported notebooks, do not load it
3. Load IPython only if there are magics in the notebook

I'd also like to possibly consider implementing the `!` action (which runs things in a shell) natively, so we don't have to import all of IPython to get just that behavior. Also investigate *why* startup takes so long!